chemical sizing is fairly accurate if verified over several additions of several different chems.
Pool Calculators are not very accurate because they don't have enough parameters entered (slopes, curves, radius', steps, etc.) and because most people tend to put the depth of the pool in and not the actual water depth.
Pool Stores like to exaggerate the size of your pool because their software figures doses (and therefore chem sales) based on the entered size. I have one PS that says my pool holds almost 16,000 gallons and it doesn't even hold quite 13,000 gallons.