Virus scan went from 99% back to 13%

I was doing a full virus scan on my PC, started at around 14:30pm and around 19:00 hours it was on 99% for a while - it now has gone back down to 13%!

Is something broken? How can I resolve this?

EDIT: looked at scan history, it says it completed after 5 hours but looks like it auto started a new one at 13% - very strange.

How to fix??