I believe we keep half the core as "proof", in theory they should contain the same amount of gold and if something happens during the first assay we can send the second ones in.

The second reason is cost, by cutting it in half we have almost the same statistically correct information, but it costs us half to assay (Of course drilling cost are the same).

Perhaps we need a minimum diameter to drill, so we use that, but we don’t need that much to assay so we don’t (To save money). If you think about it to drill down too small would probably cause things to collapse in on its self (I don’t know, I’m just typing out loud here to see what other think).