Restating, somewhat .. standard 6 adds a cog, with the same spacing,
so frame spread went to 126.
7 speed, thins the gaps between cogs, to get the extra cog in a 126 frame

Sun Tour did that with the 'ultra 6' to get another cog in a 120 spaced frame's wheel.

8 speed was adding another similarly spaced cog on a 7 speed, spread went to 130.

it's stayed at 130, for the additional cog cramming in, things got thinner yet.