Why does my utorrent download percent decrease almost every time i shutdown and start my laptop?

I am downloading a 33.15GB series using utorrent3.0. It had downloaded 2% when i restarted my laptop for installing windows updates. Then when i started utorrent, it began checking the file and when it had done, the download percentage reduced to 1.3%!!!!!!!! I can't figure out what has happened but it also happened once when it reduced from 0.7% to 0.5%. And now it happened today again. Can anyone please tell me why it is happening?