I don't have a definitive answer but since no-one else wants to...

I don't have a definitive answer but since no-one else wants to have a go I'll offer a theory.

I once had a sell on 5000 something or other which was good for day. 4980 got taken and the order terminated at the close. I was stuck with another commission if I wanted to sell the 20 the next day. My best action would have been to take what I could get for the last 20 in the matchout so that it would remain a single trade. I suspect that is what's happening on most of those odd trades.