This is likely just a case of which words the particular writer picked to use, since there's very little difference in the outcome.
However, since there is a little bit of difference given that "very hard DC for a level that's twice the ritual's spell rank." is usually 1 point of DC higher (2 points at ranks 3, 6, and 9) than it would be if the text instead said "the typical DC for a spell of its rank adjusted by a very hard modifier" there is some potential for it to have been a deliberate thing to make the DC just that extra little bit higher as part of offsetting the potential modifiers.
Though typing out that last paragraph and having to actually try to work out how to word the sentence setting the DC by mentioning the usual DC by spell rank and still not liking the phrasing after settling on something, I'd say my money is on the author picking the wording they did for the sake of it having been the easier way to communicate clearly and cleanly.
