I don't know for sure, but I wouldn't mind betting that it has to do with the traditional overheads associated with the use of hashes - in this case, hash lookup c/w direct code.

As I say I don't know, but a pound to a pinch of sh!t that one of the more learned brethren will put me (rightfully) in my place.

Update:

I should have realised from my initial observation that the difference could be at least in part put down to the difference between run-time and compile-time machinations ... as since pointed out elsewhere.