You made that as a statement. Whereas in reality it can be an influenced by availability rather than it's any kind of set rule. Take for example, random 4 letter dot coms, I'm guessing around 97% of these registrations are held by domainers Does that make them more desirable or valuable - just because you'll have a hard time finding any available to hand register.
I will add, I have managed to sell a few early 3 letter (word) dotcoms with the number 4 attached at the end. Each had the letter 'E' as the second letter - I just liked the patterning at the time (long before I learnt the Chinese don't like the number 4)
So if your thinking along the lines of random 3 letters with a Single digit number as a investment strategy. I think you'll be waiting a lot longer than the four random letter investors to see a decent return.... Yes, some will sell but that doesn't make it a good overall investment.
Yes of course I understand your numerical logic of
26 X 26 X 26 X 26
V
26 X 26 X 26 X 10
But not many company acronyms or indeed word terms have a number attached which is partly what the 4 letter investors are aiming for including the scarcity created by the registrants themselves, Somehow making a Scarcity = Value argument