Bill Zebub
“It’s probably Matt Mercer’s fault.”
This seems to ignore other mods that move the totals past 20.
I get what you're saying, but that's already taken into account.
Let's say you are attacking AC 16. If you have Str 14 and PB 2, you will hit on any roll 12 or higher. That's 9/20 odds.
With an additional +1 you hit on an 11 or higher. That's 10/20 odds.
Now let's say you're still attacking AC 16, but you're 9th level, with Str 18 and PB 4. Now you hit on an 8 or higher, for 13/20.
Again, if you add +1 you hit on a 7 or higher, for 14/20.
In both cases, your odds of hitting when up by 1/20.
But what that does mean is that relative gain gets smaller. 9/20 to 10/20 is an 11% increase. 13/20 to 14/20 is a 7.6% increase.
(Then the actual increase is damage is higher, because all of those attacks do one additional damage.)