Got any source for that? Everything I can find (and the intuitive explanation of it) points to the opposite: SPF is how much of the UV blocks, not at all how long the sunscreen stays on your skin (which varies wildly with what you're doing).

https://cdnmedia.eurofins.com/apac/media/609921/spf-iso-prot...

That's my entire point. The way they generate SPF measures how much of the sun it blocks in the lab shortly after it's applied. That one blocks 97.5% and another 98% is meaningless for the real world.