Because the official game stat formulas are not made public, these calculators are likely to be off by a few points more or less
When adding up your stats, don't forget to include buffs from any auras you may have (such as dragonlord/exalted dragonlord aura, garanak set aura, etc). If the result you get seems higher or lower than you would expect, double check to make sure you don't have any skills active, and that you didn't miss any buffs on your equipped gear. It is quite possible that these calculators will not be 100% accurate, but they should be very close...any difference of more than 5-15 points may be an input mistake
These formulas are available and accurate thanks to a lot of research and calculations by dozens of players over the years. These calculators wouldn't be possible without them.