I had a similar experience, using both the AA on my card & the in game AA at the same time. :-?
Since it has been accepted as gospel that FS AA is a no-no, I've kept quiet about it and just enjoyed it.
Maybe the technology has finally caught up with FS9's AA and finally allows it do function correctly...
...or my hardware is so outdated and stressed out that it needs both forms of AA to get the job done.
I will be interested in reading other opinions on this.