Why do some usernames have over 10 characters .-.

They don`t. Pretty sure the characters are limited to 10.

It sometimes does http://forum.toribash.com/member.php?u=4684210

there are some cases of username who has 20++ word on it
2007 account
same like above, an account from 2007
pretty recent account from 2015
Whoops, this is the most recent one: 2018

I am guessing 12 is the max.
As for why, this is just an educated guess from me. The word restriction for username may got shortened over time to reduce the annoyance of that longass name

if you make an account via the toribash website the max is 12 if it's ingame it's 10

You used to be able to make accounts with 3 characters or up to 20 characters via the in-game registration, I know because I registered both, a 3 character account name and a 20 character account name in 2015, I don't believe you can do that anymore.