Normally, the time on the front pages is displayed in "secs", "mins" and "hours". However, if there's activity on a question, and you click the little bar at the top to refresh the list (without reloading), they're suddenly reduced to "s", "m" and "h":
The state on the left is restored once you reload the page. The above screenshots are from a beta site (CodeGolf.SE), but I've confirmed that the same happens on SU and SF (so I'm pretty sure it's a network-wide thing).
I tried finding this bug on meta, but all I could find was the same behaviour on profile pages which was fixed a couple of years ago.
(PS: I've noticed that there's some odd rearranging of content upon reloading an auto-refreshed site about two days ago and have since been trying to figure out what exactly moves on the page. :D)
Edit: There also seem to be a few other things which trigger the change from long to short format. I just had SU and SF open in tabs in the background, with the long format when I left them. When I revisited them after a few minutes, they had changed to the short format.