Well, then how about this: People with similar qualifications and capability should have similar salaries for similar roles and responsibilities.

How about: People with similar performance should have similar salaries. I've seen qualified and capable people add no value, and less qualified people hustle to perform well.

That's a good point. Results are what matter.

Results are hard to quantify. You can jump on a successful project and skew your contribution factor. But what if you are working on important tooling that requires difficult problem solving. This important tooling might not be quantifiable directly like, say, an advertisement framework. It may, however, be more valuable in the long term.

I agree. I also agree that using a spreadsheet to determine this is quite difficult. I'm not saying that Google is in the right here. I'm just saying that Google's own metric for software engineer levels is not a statement of ability.