The thing is although the Xbox GPU is more powerful most of that is in extra units (increased parallelism) but overall it has a lower clockspeed, that means if all of the units are not being utilised it will be slower. It's like comparing a 3ghz 8 core processor to a 4ghz 6 core processor, if you run something that only really makes use of 6 or less cores then the 6-core processor will be faster.