The thing is for NPCs "base rate" is total rate for all intents and purposes most likely. Its not like their XP shinies / lootboxes etc are tracked.
So sure, Itachi can have his 8 XP/day or whatever, but in all likelihood that's not his base rate, because as I pointed out above, if it were, then after throwing in all the shinies & lootboxes he'd logically be able to secure, we'd be talking about him having nearly 60k XP at 22 yo, meaning 10 years after the Academy. Orochimaru is 55, which means he's had 43 years, and is probably about as talented. And sure, stagnation and all, but still.
Lets put it this way, even if you stagnated at 30 yo and don't grow one bit after that and graduate at 1k XP, 8.0 XP/day would give you 54k XP. That's from base rate alone. Throw in shinies and you could easily get over 100k. And its highly improbable that 30 yo is anywhere near the stagnation point, after all we know Jiraiya was still XP neutral at 54 yo.