'2nd Prime' sounds better than 'Short 3rd'
Although it's a small step from 2nd, it is usable and under certain circumstances, useful.
Because of this small step, I'm sure they were reluctant to include it in the step sequence as a 'real' gear until pressure came from the Marketing departing to have a 6 speed trans.
Personally, I find it rather amazing that in this day and age we have a transmission with 6 speeds and the bottom 4 gears are not spaced better. May be this the result of a new breed of engineers that have come from the early 'video game for entertainment' era......but that is such a cynical thought.