It is because it is a poll. The time stamp just represents when that thread was last updated ie. someone added to the poll and didn't necessarily contribute to the thread
I have noticed that sometimes the timestamp on a forum showing the last post does not match the timestamp in the post itself.
E.g. Take this thread: Body fat in the diet and nutrition forum. The Last post time stamp details are :' Today 02:31 PM
by Emma-Leigh Go to last post'
But the time stamp in the last post shows as: '01-22-2005, 02:11 AM #134
A significant diff of about 4 days in this case. I have noticed that this is mainly when the thread is a poll.
Ah .. ok .. thanks.