Let me try to answer your question, in broad terms.  I think it's because they are part of two different groups, and one is about to be left out in the cold.

Marten Falls appears to be more business-friendly with respect to development of the RoF, and to working with the Wynne government.  They have a known option about an access road coming up to their community from the south.  They also MAY have an option of access coming in from the east, which hasn't been officially discussed in public yet.  However, the concept of access from the east was brought up in the past with respect to the possibility of a FN-led power consortium running electrical infrastructure into the Ring (see some of Babjak's recent posts, all the clues are there).  If that happens, then there may be two separate possibilities for connecting Marten Falls to Ontario's provincial highway system.

Both of the above access points would be geographically situated in the proposed Mushkegowuk riding.  The Mushkegowuk is an association of Nations in the eastern part of northern Ontario.

The Matawa is a group of nine nations which is more to the west of northern Ontario.  Marten Falls has traditionally been part of the Matawa nations.  This group is mostly located in the proposed new Kiiwetinoong riding.  Marten Falls is the only Matawa nation which is not located in the new proposed riding.  The much-discussed "east-west road" is in that territory, if I can presume to pretend there is a final route picked for it.

Here's a map of the proposed realignment of ridings:

http://thefirstnationscanada.com/2017/08/far-northern-ontario-provincial-ridings-to-be-doubled/?doing_wp_cron=1503626142.9330050945281982421875

The "access road into the RoF," technically speaking, is a hypothetical road that goes right up into the Ring itself, ie. probably to Koper Lake.  There is no firm definition for that, at least not yet known to the public.  Separately (although presumed to be tied together) there is the discussion about roads connecting specific FN communities to the provincial highway system, ie. the road proposed on this map, which connects to Nibinamik and Webequie:

https://agoracom.com/ir/Noront/forums/discussion/topics/695950-the-east-west-road-maps/messages/2166672#message

It seems possible that the government talked with Nibinamik and Webequie, and has come to a tentative agreement about connecting their communities.  The two other Matawa communities in close proximity (Neskantaga and Eabametoong) appear to be less receptive to negotiations about a road.  The chief of Neskantaga especially (Chief Monias) has traditionally seemed to be very unreceptive to development, and is usually a fairly confrontational figure when dealing with Canadian politicians.  It appears that Wynne probably decided that Webequie and Nibinamik are receptive to roads, and decided to work with them unilaterally, whereas Neskantaga and Eabametoong were not interested in playing along with plans.  So it may be that Wynne decided to leave them out in the cold for now.

The assumption that a lot of people made after last Monday's press conference in Thunder Bay was that the extension of a road into the Koper Lake area would come from Webequie.  But that was never stated explicitly by Wynne.  And that may be why the Matawa nations appeared to suddenly backpedal & disavow.  It's possible that an end-run is happening around some of the nations within the Matawa group.  What if Ontario says, "Sure, we'll build a road to Webequie and Nibinamik, since your two nations have been cooperative.  But by the way, we're not actually interested in accessing the Ring through your territory, sorry.  We're coming in from the Marten Falls side."

Marten Falls is the key here.  They've been part of the Matawa group.  But they presumably also want control of the Ring, rather than Webequie getting control.  The [power] struggle is real.  The simplest way to do that is to splinter off from Matawa, and to work with the Mushkegowuk group and with the current liberal government.  There is no indication that this has actually happened (yet).  But it would certainly be a bold move, with great rewards for Marten Falls.

I have no doubt that there will eventually be a road directly from Webequie to Koper Lake.  The distance is relatively minor.  It only makes sense.  But the Matawa Nations effectively lose all their bargaining power if a separate road goes into Koper Lake first, from a different direction, with the support of the Nation through whose territory that transportation corridor (and possibly electricial structure) passes.

Last Monday's press conference seemed to be a bit underwhelming.  The market didn't really bite.  I wonder how the market would react if someday we get a separate conference stating that Marten Falls and Mushkegowuk are fully committed to working with the province and are about to fast-track an electrical transmission line and road access into Marten Falls and then into the Koper Lake area?

One can dream ...

Incidentally, I did something today that I should have done a long time ago.  I printed a map of all these Nations, along with clear spellings of their names, and posted it in my office.  I think it is a respectful thing that many of us investors from elsewhere in Canada should try to become more familiar with the northern Ontario FN geography, proper spellings of Nations, and culture.