I have exactly the same easel and they are nothing short of brilliant.
I must admit, I've never paid any attention to the slot length. I always slotted the paper in and slid it left, seemed logical at the time, and I've never done any different.
As to the "why?", unfortunately, I have no idea real idea, however I believe paper in rolls was popular with pro's in the past, Also for colour. So, perhaps when trimming in the dark, it gave them a bit of leeway for handling.