I think it was done to avoid needle flicker at speeds lower than 5 mph.
Iirc, Lancias of the same period were the same?
Always seemed wrong to me, nearly every other car's speedometer starts at 0.
Maybe it was done for 'styling reasons'. Fiat tried a lot of this crap back in the day. I don't know if you got the Fiat Ritmo (Racing)- a hatchback contemporary of the X1/9. This had had interior door lock buttons that rotated revealing red (locked) or green(unlocked) dots -(reason given by Fiat was to be 'stylish' and prevent theft (coathanger couldn't grip the 1/2 round button) and exterior door handles made entirely from plastic and were round - and unnatural to grip/use - (reason given by Fiat - of modern' style').
In the late '70's and onwards, it appears Fiat let the 'stylists' take control.....
AL.