That’s an arbitrary number based off of some black box calculation.

It's not arbitrary. It's the same 55/45 split that creators have gotten from ad-revenue as part of the YouTube Partner Program. I can't seem to find a source to prove it, but IIRC the split percentage has remained completely untouched for a very long time, maybe even since YPP was originally introduced in 2007.

I should also stress that this is a revenue split, not a profit split. Youtube pays all of their operating expenses after creators take their 55% share. It means that the final balance sheet for Youtube works out to something like (fudging): 55% creators, 25% expenses, 20% profit. I won't shill for the shareholders -- the deal could be better, but it's not exactly highway robbery, either.