I can’t explain that particular listing, but I have seen houses that have a pretty big spread in their “value range” listing (ha!) where they adjust the lower end up.
For example, a house might be listed between $849-$999 and they’ll change it to $899-$999. I suspect they may be getting offers at or below the bottom end of the range and since they’re not serious about it (i.e. they really want $999), they’ll just raise the lower end.
Unfortunately, ziprealty doesn’t keep track of changes to the whole value range, just the lower end. As a matter of fact, I’ve seen people raise and lower the upper part of the range and zip doesn’t track this at all.
On the Simsbury listing, it could be that they were just never serious about the $599 price. It was pretty short lived.