Weird, the Last Post column is not entirely accurate. I use it to judge if someone "else" has responded to one of MY last posts & sometimes it says I was the last poster... so I don't go into the thread. However, if I happen to open one of the threads where I was supposedly the last poster, indeed I am not. Frustrating just a little as I'd (I'm sure we'd) like to be able to respond ASAP to someone when need be.
I discussed this with someone else a few months ago. I made a few tests to see why the last post was sometimes out of date and found two reasons why this sometimes happens.
1. The main page is cached in the browser. There's no expiry tag on the page so it is very likely that the local copy will be used instead of downloading a new page from the server. Pressing the refresh button will in this case help.
2. Someone has edited an existing post. In this case the user editing the post becomes the last poster but the post is not moved to the top of the list so it might appear out of sync while in reality it's not.
In your case it sounds like 1) could be the culprit.
I think sometimes I try to force a refresh from the server with a Ctrl-F5. I'll try clearing cache next time. The editing post thing sounds plausible too.