If my memory doesn't fail me, I think that at some point achievments were registered also when Galaxy was not active, that they were synchronized when Galaxy was launched (the dlls are always in the game dir).

But lately I noticed in several games, that I only get achievments, if Galaxy is running (for example Harveys New Eyes and Batman Arkham Origins).

Or has it always been like this and it's just my accountability and memory going on vacation together?

It depends on the games and the achievement triggers. Some are activated by flags when a save game is loaded, so any achievements you earn will eventually be counted. Some are only triggered if the flag is set while the achievement monitor is running. The DLLs don't collect information, they are merely instruction sets to get the information from point a to point b.