How do GC achievements work exactly?

[CakeFuryzz]

Let's say I'm playing a game offline, and I earn an achievement such as kill 200 enemies, would they sync once I finally attempt to get online? Does it matter if I'm signed into Game Center offline?

Reading some reports, quite a few seem to have a problem with Game Center so I just wanted to be sure.

[McNippleson]

They should sync when you get back online, but it's (at least for me) different with each game. Some are synced automatically, some only sync when I launch the game again and some only sync after I get a new achievement while online.

Errors are common for me though, I got 2 achievements in Pocket planes today while I was offline and only one of them was synced when I connected to Wi-Fi.