I say everyday (or at least it seems like it) that I hate those priming calculators!
There are a couple of reasons. One is the priming 'to style' part. It sounds great in theory, but say you made an English mild. They say "to style" is .75-1.3 volumes of c02. Now that may be true in a pub with a cask ale- but it's really totally flat. Most people who buy commercial bottled beers don't have a bottle at .75 volumes, as that's not true for bottled beers.
The other thing is that while one style may have your beer turn out flat, as above, another style, say a gueuze, suggests priming to 4.5 volumes.
Well, sure, a gueuze is spritzy- but if you bottle carb to 4.5 volumes you will have gushers at the least and more likely even bottle bombs!
The other issue is the "temperature" input. The idea is that the calculator will guestimate how much residual carbonation is in the beer according to temperature. Again, good in theory- but not really a factor at all and it can be confusing for people to use.
I use a pretty standard .75 ounce corn sugar per finished gallon for lower carbed beers, and 1 ounce per finished gallon for others, like lagers, and it works out perfectly. I think sometimes we as homebrewers totally overthink things and make things more complicated than the really are.