There are no sources that consider them a different model of AT-AT. Like everything else in Rebels, it looks different because of art style.
They have extra turrets (I think the rear turret is fired in some episodes). And the relevant gallery on Starwars.com:
http://www.starwars.com/tv-shows/star-wars-rebels/relics-of-the-old-republic-trivia-gallery
mentions that it's a different model.
"Art style" can only cover so much.
The AT-ATs shown in Rogue One appear to be a different design.
With its own page:
http://www.starwars.com/databank/at-act-walker
Edited by Ironlord