I think that's just the nature of Fender's, moreso the 70's stuff. All the parts were just pulled from parts-bins when needed, so the dates are all over the place.
The usual place to start is that ones bass is the age of the youngest part, ie in your case the bass would have left the factory in '75 as that is when the neck is dated.....unless it's a non-original neck. Just means the other parts were hanging around at the factory for a bit.
Si