This means they may need to send a lot of merit per post.
Alternatively merit source could send more merit per post (including post which already merited) only for posts which considered better than most merited post.
If is not fair if they send 1 merit to a good post usually but when their source is full, they send a good post 10 merit.
Merit system has never been fair and each source is allowed to have their own rules/preference.