I think this calculator might be off on it's math. If you put in 1.15 as the growth rate, you get to the monthly increase in difficulty in 2 increases, but this is 28*.85 days, right? There should have been a third difficulty increase for the last part of the first month.
Yup, he explicitly states that he calculates difficulty changes every 14 days and not depending on the hashrate. So it's even a bit worse than predicted, although I'm still not convinced that difficulty will increase constantly. It's more like a curve that gets shallower and steeper every now and then.