20 year old level 11 here :)
My guess is that level 11 makes sense to most people, because it's the same amount of numbers as a full phone number, so you can get a cadence going.
2-232-340-1010 is pretty easy to remember, because it already exists as a common cadence, even if it's not in the US format like I use.
To me it seemed like numbers which could be split into two equal groups were the easiest. 235 431, 4475 3268 etc.
I also sang the numbers in my head and tried to memorize by repeating the melody after the numbers disappeared.
There is probably a reason why level 11 seems to be so prevalent. Phone numbers could be a factor, but to be honest, I don't know a single phone number by heart, which is probably due to my young age. Never had to memorize a phone number...
The best training for me are probably 2FA codes, so up to 6 digits I should have it covered ;)
EDIT: I realize that I just called you old. Sorry.