Why does iPhone need 10% free space for ProRes video recording?

I understand ProRes is large and requires a lot of space, but the 10% metric makes no sense to me. A 256GB iPhone only needs to have 25.6GB free, but my 1TB iPhone needs to have 100GB? Why a percentage and not a flat amount?

For context, I'm on the 1TB 15 Pro Max and have my entire photo library downloaded because I hate waiting for photos to load, and they never load in full res unless you click the edit button, then they download. So I got the 1TB and downloaded my library. BUT, even though I have ~90GB free, I can't record ProRes internally because Apple decided to do a percentage instead of a flat data requirement.