The fact of the calculation being done on a different worksheet doesn't imply that the result will always be different. It's simply a worksheet that needs to be used to make sure the amount is calculated under the right circumstances. It might have produced a different number and did so for many people, so it had to be used in case it did produce a different number for you too. There are actually three different types of worksheets that can be used, and must be, if the circumstances call for it. Sometimes the answer might be the same on any two but they always need to be used, just in case.
Just for your own information, you can always use a right-click on the number to find the data source. Some fields don't offer this but, in the desktop version you're using, the data sourcing is a very valuable tool for most fields. Sometimes, when you right-click, you'll see what looks like a little popsicle appear to the left or right of the number. You can click right on that popsicle and you'll be taken to the worksheet or form that serves as the source for the calculation of the field. Other times, you'll see the Data Source choice in the drop-down.
