If some schools allow GPA's over 100%, or 4.00 then theoretically could admission averages follow suit?

Sure!
And at that point everybody would get a scholarship to Harvard and then graduate Cum Laude
Then they would get an MD at John’s Hopkins.
Then one day they would kill you in the operating room because they were faking it the whole time.
How cool is that?!

No. High schools have long provided GPA figures over 4.0. When I had reason to stay abreast of such things colleges didn’t consider anything over 4.0.

It seems to me that colleges who would require such high marks, are also ones that pay attention to which schools applicants go to, and what their grading scales are. They did that 35+ years ago, so by now they probably have really good databases that will tell them for each school (not just the better ones) what the school’s grading practices are like, what the ranges and averages are, and even how well the students from those schools have tended to have performed.

Even decades ago, it would be rare for a good college to be deceived by a school having an inflated grading scale.

An A in a high school AP class is usually weighted higher. At my kid’s high school, an A in an AP class was given a 5.0. At some high schools it would be a 4.3. As @Zaku said, colleges know what scale a school uses, and also can tell what courses a student takes.

And admission standards are higher at better universities. At the University of California, Berkeley, the average high school GPA of entering freshmen is 3.86 on a 4.0 scale.

At Stanford, the junior college in Palo Alto, the average high school GPA of exerting freshmen is 3.96

@zenvelo haha! Don’t knock my granddaughter’s probable future alma mater!

@janbb I am sure your grandkids will excel at Cal!

@zenvelo She will have her choice but I’ll be pushing for Princeton!