Going to be the spoil sport: that's a consequence of wanting the fitted sheet not to pop off or slide around. I've always just gotten the same size as my bed and found no real issues – timing from first correct corner, on in around 60 seconds. If you have a physical disability or accidentally got a sheet that doesn't match your mattress (I've heard height is usually the culprit, but no lived experience here), I could see that changing.

I tend to use mattress toppers, and the height of the mattress is absolutely a barrier to good fit. It never occurred to me to look for sheets with specifically deep pockets, but they do exist.

Flat sheet crew represent!

You know, I never even considered mattress toppers/mattress height. I appreciate the insight; I'll keep this in mind the next time I'm searching for a mattress.

Edit: Wait, I'm so stupid. The people talking about the "height" were talking about that height, not the height from header-to-footer. I'm so bad at this; I would've considered that "height" to be the depth – looking at the top of the mattress as an x–y plane.

I think the real issue is matress thickness. The width and lengh are standard, but then you start getring thicker and an added pillow top and now its too thick for the normal fitted sheet.

Mattress thickness aka the height of the sheet.

But yeah that is the issue. For me it's the other way around as I've a large single and the mattress is essentially built in. So I just have a mattress topper (actually two) on it and the fitted sheet on that. And the sheet is way too high for just the mattress topper. Or the mattress topper isn't thick enough. Same thing, really.

But makes fitted sheets run around like crazy.