Where are you getting those specs? AFAIK the 2014 MacBook Air doesn't come out of the box with an i7 4650U, but rather an 4260U, which has a Passmark score of 3617. Also note that the Apple presentation noted a Turbo Boost top of 2.9GHz, not the 2.6GHz of the Core M 5Y70 that you linked.
It's the difference between horsepower and 0-60. There are other factors involved. (This is largely academic, though, because I doubt anybody who owns one is going to really care.)
For the new Macbook you are right that Apple appears to use a slightly faster variant of the 5Y70 that doesn't seem to have a benchmark on cpubenchmark.net yet (at least I couldn't find it).
I'd be surprised if these 300MHz made a significant difference, though.
It's the difference between horsepower and 0-60. There are other factors involved. (This is largely academic, though, because I doubt anybody who owns one is going to really care.)