Of course the other reason is someone back in the day swapped the transmission (for a Berlina....). It happens a lot in Lancia land.
On all Aurelias the final drive ratio is stamped on the underside of the transaxle casing fairly close to the riveted blue plate showing the model and build number.
Might be worth a look?
Chris
Have you managed to look at the underside of your transaxle yet to establish what final drive is actually in the car?
Chris