Let me qualify what I said. Sometimes I understand how that happens, though I am far from an expert in how brokers operate.

In this case, I think all similarly priced buy orders go into sort of a pool at that share price. Then as the price is achieved, orders are filled by who is next in line. If I'm next in line, but want 100,000 shares when only 50 become available, then my unqualified order gets filled at that level - 50 shares -, and the next buyer become first in line for the next shares to become available at that price. That might be the 100,00 I wanted, but I've now dropped to the bottom of the list of those who wanted shares at that price, and my bid is no longer "round", it's 99,950 shares - "round" orders/bids are easier to fill than those that are not round.

Meanwhile, let's say, some seller comes along who wants to dump 1,000,000 shares as easily as possible. That means the deal can be done at less than my bid, because I'm now out of line, and am not bidding for enough shares at one time to satisfy the seller. ...and maybe I am not a "friend" of the seller or broker either.

These examples are overly simplified, and I look forward to what others may share along these lines.

VP