NavList:

It actually tells you in the instructions (in the 1980 epoch of AP3270 at least):

“Many factors were considered in making the selection, including azimuth, magnitude, altitude and continuity.  Continuity was sought in regard to both latitude and hour angle, particularly for latitude where changes are not immediately evident by inspection”
“A total of 41 stars is used, of which 19 are of the first magnitude (brighter than magnitude 1.5) and 17 of the second magnitude.  The names of the 19 first-magnitude stars are given in capital letters.”
“Of each selection of seven stars, three are marked with a diamond symbol as being suitable for a three star fix.”

I would imagine “changes not immediately evident” above includes things like stars close to meridian passage. Using one would make nonsense of averaging during long sandwich fixes.

One thing that becomes clear again and again is that the working party from the RAF, RCAF, and USAF who agreed the original specification for these tables in 1950 must have included some very experienced air navigators who really knew their stuff. DaveP