Could also be that appraisers are being conservative (their "customers" are the banks, after all, not the buyers) in their estimates and haven't caught up with the higher bidding?

But isn't "recent sale prices of comparable properties" about market expectations though? Like isn't the implication of an appraisal that if you were forced to sell the house you could get the appraised amount for it?

You're right, and in a bull market it's inherently conservative since you're looking at past prices. What I mean is, if houses are selling more than appraisal it's mainly because prices are up compared to recent sales, not because appraisers are accounting for any particular view about how that will play out over the next few years.

It was surprisingly subjective though. It wasn't like you were feeding lots of data into big statistical models. You'd only pick the few closest comp houses that sold recently. They'd still have different features, so you'd try to figure out the value of the features by comparing the comp houses. It was a simple mechanical procedure but you could work it out different ways and get different numbers. Our instructor admitted there was a lot of opportunity to make it come out however you wanted, and even said that experienced appraisers did so consciously.

So I wouldn't be surprised if an appraiser who thought the market was in for decline actually did write lower appraisals, though they'd justify it entirely by recent comps.