next interval >= prev interval + 1
next interval >= prev interval + 1

http://ankisrs.net/docs/manual.html#reviews

One final thing to note is that Anki forces a new interval to be at least 1 day longer than it was previously so that you don’t get stuck reviewing with the same interval forever. 

https://anki.tenderapp.com/discussions/ankidesktop/18382-new-interval-does-not-seem-to-be-updated-with-ease-at-130-and-current-interval-at-7-days

In some cases interval does not grow in fact.
Stvad/anki