Are you sure about this move order ?
Help me understand

Hey, that's true, but black always followed Bb5 with Nc6, not axb5. Maybe for a bit of fun the players decided to get into the Ruy Lopez via a weird move order, just to rattle a few cages.
If I'm understanding you correctly there's nothing obviously wrong with axb5? It's just the Game Explorer which doesn't have a good response to the previous move?
Maybe the move a6 is so unusual that the Game Explorer doesn't have enough data to make a good follow up on it?
After this opening the Game Explorer suggests Bb5. Why?
And then it suggest Nc6. Why not axb5?