That's a really good question, one that I'll bet some our veteran boardies could answer. Let's see if @bedrockcomics has anything to say about this!
Just looking at the final six issues of Capt. Aero on GPA census, it is clear that in 2019, #26 is the most prevalent:
#21 - 10 copies (Universal)
#22 - 7 copies (Universal)
#23 - 8 copies (Universal)
#24 - 9 copies (Universal)
#25 - 17 copies (Universal)
#26 - 31 copies (Universal)
The most likely reasons for a no-show, as you say, are either the book was super rare, or for whatever reason, not available at the time. Of course, the more rare, the less likely to be available, but I'm sure there could have been all sort of exceptions where it just wasn't possible to line up the owner, the book, and the photographer, even if the book was not particularly rare.
I suppose in 1989, when the Gerber volumes were published, it might actually have been the case that #26 was harder to find (i.e., "rare") than the other issues, and only with the passage of time have more copies of #26 risen to the surface.
Curiously, for at least some titles that these days are considered to be rare, there are no Gerber no-shows, such as the Centaur line of Amazing-Man, Amazing Mystery Funnies, Detective Picture Stories.
There are strange forces at play!