They give you a partial and now the order doesn't have enough shares to show. They're just playing the game; you gotta play too.
It's not really ethical in terms of giving the investor the best fill but they are trying to make gains against us.
Think about it for a sec, they have to take the other side of every trade. In theory, that should be a losing deal if investors are ever to be right.
Trust me, i'm not an MM sympathizer, not in the least.
But at least we know we have a worthy opponent. ;-)