I'm guessing it's something to do with what parts of the file are available and whilst you may have been downloading at X speed for X time causing a calculation of that downloaded amount you might yet to have downloaded all the bits you need for the file to be complete. I can't say I full understand it but I think there's some amount of redundancy happening on the downloading due to who's sharing what % themselves since not everyone has 100% and it might be you are waiting on someone with 100% having a free connection for you to connect to to get the last few bits you need off them.