Not to put words in the OP's mouth, I think he might be alluding to shot sizes being even, except for 7.5.

That is 4, 5, 6, 7.5, 8, 9.

It does stand out as an oddity and I am sure there is a historical reason, but don't know what it is.

Maybe it has something to do with live pigeon loads using 7 shot "back in the day" and 7.5 shot was used to differentiate the load?