Just maybe -

1. He thought you'd overtake on the right, or
2. He was moving over ready to move into the left lane having realised he was in the wrong long, or
3. He didnt have a clue you were there.

Me I cant help thinking life's too short to get wound up about minor things like that - 2 minutes down the road and you're gonna be nipping past anyway.

If that's the worst that happened that day surely its not that bad.