At the club levels players are often 50% on there's shot making. At the olympic levels you'll see 75-95%...
That's the point. No one is perfect. You can line up a shot, throw the same weight, hit the broom and still miss in curling.
The effects portrayed in this video game don't mimic real curling but, they are variances the Online-Curler needs to handle.
Maybe lag did it? I dunno. Either way, the most skillful survive and part of the skill is dealing with variances in either offline, real curling, or online curling.
For instance... When the weather gets warm, at my Curling club some sheets of ice have negative curl. You actually have to put the opposite turn to the way you're curling if you want to make a more predictable shot, otherwise the rock takes off in the wrong direction or massively over curls.
It's not good curling by any means, but IT IS curling.