The pool calculator website is client side scripting so you can see the calculation by viewing the source. Here's what it's doing:

parseFloat(document.F.FCto.value)-
parseFloat(document.F.FCfrom.value)) *
GetGallons() / 482.202 * 6 / parseFloat(document.F.FCpcnt.value);
document.F.FC1oz.value=PutVolume(temp);
Which is basically Target increase*water volume /482.202*6/LC strength

Perhaps the difference is in order of operations or that 482.202 constant. I'm not intimate enough with the formula to know what that is for. I wonder why they didn't use 2,893.212 for the constant.