Or every generation of programmers gets larger and more varied than the last because of increasing computer penetration (there was a time when they weren't in most governments, now they're in most pockets), the lure of jobs and exposure through school.

As your population grows, your average usually goes down unless there are more smart-and-deep people than not. That's why each generation of college grads is dumber, for example: more people go as a percentage but more people aren't super-smart as a percentage. Even if you have more S&Ds than not, selection effects might bring you down over time because they were the first adopters, leaving the remaining pool worse on average.

Never ascribe to character what can be more easily ascribed to population.