Although there are 12 different rallies in career mode you only have to win 10 of them for this achievement for some reason. It just popped for me when I won my 10th rally (Acropolis) even though I've yet to win at Sardinia or Japan. I noticed the counter for the achievement was going up by 10% for each rally I won so I knew something was up.
I think there were some last minute challenges to the rallies they included in this game so perhaps that explains it?