Not baffling at all. There is only a crime if she didn't give consent. The accuser said she couldn't recall if she gave consent or not. Given that even she isn't claiming that consent wasn't given why would you expect a guilty verdict?
Take it out of the sexual arena, say it was this:
A guy gets very drunk in a strange part of town, wants to walk home but isn't sure how to get there from where he is. 
So he asks a stranger to show him the way. The stranger tells him ok but it is going to take him a half hour to escort him there so the drunk checks his pockets. He has $30 and offers to pay the guy $10. Agreement  made, off they go.
Next morning the drunk guy wakes up and can't remember anything after the time they start walking. When he checks his pants the entire $30 is gone.
What happened? We don't know. Maybe the 2nd guy stole it. Maybe the 1st guy felt generous for all the help the guy gave him (this happens all the time with drunks - ask a pub waitress). We don't know, so no jail.