I have purged the stored posts,
Don't do that to solve this. If you purge the stored groups Newsbin will lose track of the last downloads and can pull old posts. Old posts that download and fed to the post list in real time don't get filtered.
If you want to reduce how many headers Newsbin stores use the "Storage Age" setting. That "Display Age" setting only changes the display. It has no impact on the actual storage of headers.
Assuming you have some dead groups in your book list, say groups that haven't updated in 1100 day. If you then purge the groups and download headers, these 1100 day old headers will show up because Newsbin will always download some headers from a fresh group. Purging resets the group and makes it "Fresh" again.
So, I'd suggest never purge a group unless something is wrong with it. Manage the stored headers using the "Storage Age" setting instead.
The reason there's a "display age" and a "storage age" is you might want to keep 1000 days worth of headers but only see that last 30 days normally. Then you can tell Newsbin to load all 1000 days if you're searching for something old.