This is a new glitch as of 1.09. Prior to this update, I had 100% completion of the game (as displayed in the status section). I wondered why, a while after the update, it went down to 99% completion.
I've now found out why. If you finish second in a career mode race that you previously had won, your trophy marker outside the event now turns to a silver trophy. Accordingly, the game considers you not to have fully completed the event.
Effectively, if one so desired, he could wind back your game to 0% by entering and losing every race. This is something PD really should have picked up before releasing it in 1.09.
Edit: The single race that I had been given a silver trophy for was Apricot Hill of the Standard Red Bull Championship. After trying the event again and winning that race, it has returned to gold and my status is 100% again. Interestingly, even though I lost the fourth race in this championship, the associated trophy for the fourth race remained gold.