Welcome to the joys of "round off error".
When you compute values by dividing and/or by multiplying by percent values, The result is stored in fileMaker down to many decimal places after the cents place--just like punching it up on a calculator display, but your report likely displays the values rounded to the nearest cent. That small difference between the rounded and unrounded values can add up over the life of the loan to produce a small discrepancy. If you use the round function to round each calculation to the nearest cent, I think you'll see this discrepancy disappear.
You hit the nail right on the head. Thanks again PhilModJunk!
Can I just be the first person to say:
84 month auto loan!?