Edit: it looks like "X minutes ago" feature is broken. Tooltips show the correct time, which is more than 10 hours ago, while it shows 15 minutes ago.
When this happens, the relative timestamps (X minutes ago etc.) are reset to the time at which the post is selected for the front page. The absolute timestamps in the tooltips, like you have discovered, show the actual time in this case.
Edit: it looks like "X minutes ago" feature is broken. Tooltips show the correct time, which is more than 10 hours ago, while it shows 15 minutes ago.