Happens all the time. They may move up to grab a lot or down to sell a lot just to provide liquidity - in other words, they may elect to take the other side of a trade in the middle of the spread.
Additionally, I would assume that their platforms allow for two sets of numbers - their "advertised" bid and ask and then the "real" bid they would fill (possibly higher) and real ask (possibly lower) they would sell for. That is certainly the way I would program the system and it took about 15 seconds to think of that design.
It is exactly the same as when you sell a used car. You might list it at $10K, but know all along you would take $9500 in a heartbeat.
This is not evidence of a conspiracy or manipulation despite what many say.
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