Right on; within a narrow range these loads all have near the same column length in a nominal size bore. It is also noted that these are pretty much what the standard loads were a decade or so either side of 1900 when powder was limited to black or the early Smokeless which all burned at similar rates.
As stated in a previous post it was the development of progressive shotshell powders beginning in the 1920'that allowed the smaller bores to be "Loaded Up" to heavier charges.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra