You're confusing "The Exception That Proves the Rule" (in English, as used colloquially) with "exceptio probat regulam in casibus non exceptis" (in Latin, which has a use similar to what you're describing.) While the law attempts to be precise, common usage embraces ambiguity.
> exceptio probat regulam in casibus non exceptis
Or more succinctly, as first-year law students learn: Expressio unius est exclusio alterius — to state one thing is to implicitly exclude others.
https://definitions.lsd.law/expressio-unius-est-exclusio-alt...
RIP modus ponens!
They really mean the same. What changed was the meaning of the word "proves" in English. When the saying was coined it meant "tests", not "confirms". People kept saying the...saying even though they were using it backwards.
Which is amusing because “proving grounds” is still using the old definition of “proves” :)