Same, I find this to be a bizarre practice. What's the point if the snapshots are revised?

I assume the rationale is that it's are supposed to be an indicator of the actual value of the property at a point in time, not the accuracy of their algorithm at that point in time. So if they update with new information, it's assumed that instead of the price suddenly jumping (which looks very odd if you're trying to understand a local property market), the original snapshots are assumed to have been wrong

At least in my area, they've completely given up on providing a "historical" estimate of the house in question and just have a trend graph for the general area. I assume this data is also heavily massaged, too, though.

I am guessing they don't save the price history as snapshots. They have the current estimate, and then they walk it back in time from the market averages.

This happens all the time with most data providers.