Recently, I played a game and after running the computer analysis, it seems like playing 5. Ba4 was a mistake, being better Bxc6.
However, I cannot find the reason why. By retreating the bishop, White does not lose a pawn after ...Nfxe4. So I cannot see why my move would be a mistake.
The difference is that after Bxc6 you win a pawn with Nxe5. That's why the computer gives a +1 evaluation for it.
After Ba4 you end up with just an equal, normal position.
So it is rather the difference between going from better/won to equal than from equal to worse.
In fact 4...a6 is a weak move by black. Most popular and good is 4...Nxe4.
