Or a delayed reporting engine.

It's possible that the buy/sell are a little out of sync if the orderbook is deep enough and there is enough activity.  Whereas the matching engine would "one would hope", freeze the DB long enough to match and then release.

One very solid and scalable design for an exchange involves all limit orders going to the orderbook, then the matching engine locking the books temporarily, taking all market orders and matching them first, and only then matching limit orders across the books.  This might result in what you're seeing.

Since you're only seeing limit orders in the order books and last can be either market or limit.
It would resolve itself the next time the matching engine takes a pass across the orderbooks.

Seeing that is a good sign that there is pent up pressure in the market in one direction or the other and would likely mean it's a good time to take a contra position.  Just an FYI for any bot makers out there.