Just curious if you hit a 3-wood off the tee at all during this round?
It's only a drive with a driver that counts towards your driving accuracy stat, whereas I'm fairly certain a 3-wood still counts as an attempt. In other words if you hit all 3-woods off the tee, and hit every fairway, you would have 0% driving accuracy, since you technically didn't hit any "drives" (by the WGT definition of the word, anyway).
In your case, based on genorb's calculations, my guess is that you hit your 3-wood off the tee on one hole and your driver on the other 6 (not including the par 3's).
And yes, it is indeed a minor flaw in the game (believe it or not....they DO exist! lol).