i don't know why brewcalcs.com is way off, but there's a little bit of discrepancy to be expected between various software because of rounding error. The 131 multiplier that we all use is a rounded number.
My guess is that its not really linear like in revvys formula. That the formula is close enough for a certain range but the computers have more complicated algorithms to find it. However I may just be talking out of my ass.