They are not overdrives, they are gears that sort of come between the other gears to make the acceleration faster and smoother. Saab calls these gears 2,5 and 3,5 i.e. they come between 2nd and 3rd gear and 3rd and 4th gear respectively.
Which is the same as the sentronic auto 'box on the 9-5.
In a manual gearbox, it's possible to have 'overdrive' ratios (4th and 5th) to give the low revs at cruising speed, but the acceleration is from the lower (numerically higher) diff ratio of around 3.5.
On the autobox, the top gear (5th) is effectively direct top, not an overdrive, which means to get the same low revs at cruising speed the diff ratio has to be higher (numerically lower) and is 2.44. But, this would mean the car would have very poor acceleration, so the intermediate gears in the gearbox have a much lower ratio to allow the car to accelerate and overcome the high diff ratio.