no reason, the entire Seio line from the 620 gph to the 2600 gph models all work on a similar high flow but more diffuse idea, which also is a lot more energy efficient as well.
The 1100 model, simply is more flow than the 820 model, it also is a larger pump inside the tank, I never liked the vertical mounting idea because the water goes through a 90 degree turn to get shot out and that can't be that good, so can't comment about how well it works in the "cleaner" mounting fashion.
