

Maybe you’re at 99% because you’ve been yawning?
Ok, I am sufficiently caffeinated and bothered to look up a study: https://pmc.ncbi.nlm.nih.gov/articles/PMC3678674/
According to the above, they suspect it has to do with helping to keep you relative alert/awake but also, possibly to cool the brain and adjust ear pressure.
Yeah, there doesn’t seem to be anything (at least in that study) to suggest it actually impacts blood oxygen levels either.