Exactly why, I don't know. But a fair guess is that it either had to do with smog specifications or to compensate (idle in particular) for the awful smog devices of the day. It might also be how the timing was measured. There is  possibility that certain cars were set with the vacuum advance attached and that would elevate the timing at idle (hence the 6 vs 20 degrees) but upon acceleration timing would drop.

When one observes the smog devices of the 70's/80's it seems as if there were teams assigned to certain RPM ranges each designing a separate device and then they were just thrown onto the engine thinking it added up to a collective whole. I'm not saying it was that way, just that the end result has the appearance of such.

 Even the OBD I injection was a significant advantage over the carburetors of that time era. Some here have gone to the add on injection systems* while others have just adapted a complete engine swap from the 80's/90's Mustangs or Rangers.

*