If my memory serves (and it may not) this is not an issue with Inventor, but with all computers. When using binary (1's and 0's as all current commercially available computers do) to do math with decimals it gets tricky because some numbers can't truly be represented. If you run the same math in another software (say Excel) and show the the number to the 14th decimal you'll find the same number. Google "floating point rounding error" if you need proof (that's my why of saying I don't truly understand it but have accepted it as true.)