There could be All Or Nothing sell orders that were undercutting the visible Ask. They are always invisible and don't require special MM tricks or anything. Any retail can use them. I love them myself, as you don't end up tripping over each other trying to be the best Bid or Ask. You just sneak in invisibly. The downside is you can get skipped over if your order size is too large. The counter to that is putting in multiple smaller orders to hopefully get hit.
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