pcgs weightings: how are the numbers(1-10) dtermined for the different registries?

1907Quarter

I find the whole thing rather confusing in that there are some coins which are plentiful in some grades while rare in lofty ones, and this can't be accurately reflective when comparing one coin to another in a particular registry.

Also, can I assume a number(1-10) reflects it's relative importance WITHIN the PARTICULAR registry and not across ALL the registries?

Hope I'm not missing something basic here-I'm new to this!