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> What's puzzling is that they also bumped up the historical Zestimates

There are two things a "historical estimate" could mean:

1) What did Zillow say the value was on day X?

2) What does Zillow now think the value was on day X?

You and others seem to be thinking of (1), but I think Zillow actually is doing (2). Since the goal is to figure out what the house is worth and how its value has changed, I think (2) is actually the right metric here.

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(2), of course, has the advantage of obscuring past mistakes and producing a nice smooth chart.

Unless you think they should never update their algorithm, approach (2) is just making the chart consistent with itself.

The useful part of these charts isn't really the current Zestimate, but rather the trend line of the home values, especially at the neighborhood level.

If you don't recalculate the historical values, then you break the ability to see the trend line.

If you really want to see the results of previous models, then the way to do that is to plot a separate line for each model, but you can see why they wouldn't want to do that from a product perspective.

Every county in the US that publishes historical valuations seem to get by without overwriting history.

Yes, but those are actual valuations based on sales prices, not predicted valuations based on an algorithm. Different thing.

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