Well when the 13 Episodes series started the manga also had just started. So along with altering some episodes they used filler episodes at most.
The Ultimate Ovas' are made now, and the manga has progressed a lot, so if it continues like this it's going to be a great success. It's still a bit different but only because it's Ova and hasn't enough "space" for everything.
Unfortunately the manga Still has not ended. So the end of the Ovas is still uncertain.