[Flaropri]

Speaking for myself, it can take a while to get used to aiming with a new game. For example, TF2 tends to have either slow-moving projectiles (relatively) or hit-scan... or flamethrowers I suppose. Anyway, the point is that the weapons both feel and function differently than in, for example Crysis or Tribes, and those games in turn feel quite different from each other as well.

It just takes time to adjust, and part of that is also different default mouse sensitivities or what have you.

I'm notoriously bad for "leading the target" as a Sniper (completely counter-productive for those not familiar) in TF2 or just "needing" that extra half-second to confirm they are still in my sites. I also haven't spent nearly enough time playing Sniper in that game to really get used to it compared to Soldier or Pyro.

It's also like this in other games, some with even smaller differences. I'm relatively used to last-hitting in LoL for example, but if I switch to HoN, RoI, or DotA I tend to miss more often for a while just because of the different balancing and slight variations in interface.