So I presume Suu went with 5 because 6 is kinda difficult to do and author rolled with 59 instead of 69.
In Japan they have number jokes called goroawase, meaning they can have so much more fun with numbers than us.
95 can mean "moaning" (with pleasure?) and 59 "face sitting" (meaning 95 must have been about the orgasm if you can't get there without a 59 first)
74 style face sitting seems to be able to mean "hole teasing" while 320 style could possibly mean either "rough surprise" or "enjoying eating a beautiful woman" - and 8 means "perineum" meaning more butt stuff.
So the jokes behind the numbers are not as chaste as just "almost a 69", they are a lot lewder than that.
Source: crappy MTL of the excellent page
https://seoi.net/goro/