Why Are The PCGS Variety Listings All Different?

I have been trying to figure this out for a while, and I'm still not sure I've got the complete picture.

If you go to the PCGS price guide for a particular series, there are some variety drop-downs under the main coins. I assumed that was a complete listing of attributed varieties. However, in browsing many individual coin pages, I have learned that the CoinFacts drop downs for varieties are often much more extensive than the price guide. One reason why varieties don't appear in the price guide vs. per-coin CoinFacts seems to be whether they have any price guidance or history for the variety (and that seems completely fair, they probably don't want to list blank entries in their price guide table). But that isn't consistent.

To my further puzzlement, in Google searches for varieties, I occasionally turn up PCGS slabbed coins with CPG attributions that do not appear to have CoinFacts landing pages.

Aside from the super-vague coin submission variety rules, is there any proven method to get a solid list of PCGS varieties for a given denomination? Or is that just something one needs to cobble together? That task is made much more challenging by the lack of complete published CPG lists for modern and recent vintage coins.