Those who come after are selling cheaper.
That’s the only way they would sell before you.
No, there’s definitely something wrong. I posted a stack of 250 copper ore up for 21c each about 20 hours ago (screen says 4 hours ago though, which is a different issue). About 8 hours later, I posted another 89 also at 21c each. 89 copper ore has sold, one in a transaction of 87, and one in a transaction of 2. Exactly 250 remain. I then also sold a stack of 20 copper ore for 21c each, as well as a 10 and 28 stack. Still, 250 remain that won’t sell.
This is the second time I have noticed this, something is wrong.