Shimano 7-speed cog spacings are 5mm center-to-center, EXCEPT for the 2-3 spacing (between 2nd and 3rd-smallest cogs) which uses a 3.3mm black spacer instead of the other position's 3.1mm brownish spacers.

This was done to extend service intervals.
As GrayJay points out, a dirty cable won't allow free enough movement as the spring relaxes, so the slightly-longer cable travel of that indexing click improves shift movement resolution at the derailer.
The 1-2 shift (smallest cogs) also has increased cable travel from the shifter, but you limit it as needed with the hi-limit screw.

One consequence of this asymmetric cog spacing is that if you combine a rapid-rise rear derailer with a normal (not designed for low-normal) Shimano shifter, the indexing will be way out of whack unless you put the thicker black cog spacer at the other end of the cassette. Likewise, the shifters Shimano offered for use with low-normal derailers won't index properly with a normal (hi-normal) rear derailer without, again, reversing the cassette-cog spacing.

Beware that some cassettes have a partially built-on spacer on the 2nd-position (2nd-smallest) cog, combined with a ~1mm steel spacer, so spacing corrections/changes can be difficult.

And also note that IG cassettes use thicker cogs with much-narrowed spacers to achieve the same 7-speed spacing as ALL other Shimano 7-speed cassettes and freewheels.

Suntour's 7-speed freewheels and earlier cassettes used their own version of asymmetric spacing, which extended across the range and used much-thinner spacers between the biggest 3 cogs. This is why their freewheels don't work well with Shimano indexing mech's and why these freewheels work so much better with 9-speed chain in either friction or indexing mode.
And, due to Suntour's thicker, blunter cog teeth, even their 6-speed freewheels shift much better with 9-speed chain, since these narrower chains suffer much less tendency to overshift toward the bigger cogs.