Sam had no way of knowing what happened “before”, since it’s impossible to remember things that didn’t happen yet.
He went back in time alright, but he couldn’t change stuff he knew nothing about.

His car’s OK at the moment (although it cost him an arm and a leg to fix), Amy’s still here (if you ask him, he’s not so sure it’s a good thing). All in all, he’s doing much better than he will a month from now.

He doesn’t know it yet. As far as Sam is concerned, he’s having a crummy day, but let’s go back to the beginning, a month later.