The speed is because, in AWS, all planes of the same fleet type must go the same cruise speed. It's a limitation of the scheduling design but it rarely amounts to anything worthy of note.
Not sure on the seating. I'd have thought it would have come up previously, but I don't find any discussions on it. I don't think it's supposed to be exactly what Boeing says here, but it should most definitely be more seats than the -400ER for certain, not quite a few less.