SPF rating is subjective at best and not a regulated term in many countries.

Australia is a place where it is a regulated term and a test by Choice showed that 16 of 20 products failed their rating and several have since been recalled.
https://www.choice.com.au/health-and-body/beauty-and-personal-care/skin-care-and-cosmetics/articles/sunscreen-test

Back to your original question which is answered in the article

In percentage figures that means the difference between, say, an SPF 50 sunscreen and an SPF 30 sunscreen is less impactful than you might assume. An SPF of 50 protects you from all but 1/50th of UV rays (98%), while sunscreen with an SPF of 30 protects you from all but 1/30th of UV rays (96.7%).

SPF50 means that only 1⁄50 of total UV light gets to your skin. If the SPF50 factor for a narrow light wavelength was tested under a 20 μm layer of sunscreen, and you'll manage to apply a 40 μm layer, the resulting UV light would be 100%÷50÷50=0.04%, corresponding to SPF2500 for this wavelength.

Of course, SPF is not measured for one particular wavelength. UV light is a spectrum of sunlight of wavelengths starting at 100 nm to 400 nm. UV radiation of shorter wavelengths contains more energy, but is easier to stop; it usually responsible for short-term damage like sunburns. For longer waves it's vice versa, they are less powerful and won't cause a burn, but are harder to stop and penetrate skin deeper and cause cancer and premature aging.

Low-SPF sunscreens are usually achieving their index thanks to stopping easy-to-catch short waves that are most of UV exposure in pure joules, and are having a hard time stopping long-wave UV due to their chemical composition. For further reading I recommend a 2019 article “Critical Wavelength and Broad-Spectrum UV Protection”, Google it, I'm cautious to paste a link in case anti-spam bots are tightly configured here.

SPF is overall not a very good and all-encompassing sunscreen quality indicator. I recommend looking into UVA-centered indexes: CW (over 370 is good, in EU these are marked with UVA circled mark), UVAPF or PPD (over 15 is decent, that's same system as SPF but for UVA, longer waves) or, common on asian brands, PA++… index (UVAPF=2^x, where x is the amount of “+” symbols after PA).

If you've ever used a UV camera it's pretty clear that much like lotion not absorbing after a certain point, there's a limit to how dark your skin gets with a given sunscreen. Also that there's a massive difference between brands and formulas. I had some photos I took with one but can't find them. If I do later I'll update.

Searched but couldn't find them. Did find a conversation though where I used it to show most cars in a parking lot have UV blocking windshields but every other window is transparent unless they have tint

Think of the visible spectrum of light. If you were to mix a 30% shade of white with a 30% shade of white, you get…. A 30% shade of white.

Now just switch that coloring to the UV end of the spectrum and it’s the same thing. You’re not layering it realistically, you’re basically just mixing it together and the result is still the same “color” you started with.

Don't buy American sun screen. It's always cakey and flakey. Go import some Korean sunscreen. It's basically feels like you have nothing on and gives you better protection.

The store increases the price because people will pay more for higher number. You can get spf 80 or 100 cheap elsewhere.

By god I think you've cracked the thing.