The proper set-up for a jointer is to create a concave edge over length, as is also the case for a hand plane, but the spring should be very slight. Assuming everything else is equal, you can get a change in the cut as the blade dulls, this can work in different ways depending on how high the blades are above the outfeed to start with.
The comic book view of how a jointer works is that a perfectly straight edge is fed off a perfectly level infeed to a blade that is at the height of the outfeed so that a thickness of wood is removed. That is thicknessing, and smoothing not jointing. And given that a lot of the wood that we get is pretty clean to start with, it will work well enough. I prefer to set my jointers up to work from the center of the board out. That may be why the European jointers have a gap. I have owned several of both types and I have kept my General so I have to deal with the swinging guard. Either way I work out to the ends, with a set spring that I can always clip off at the end if I want to. When working from the center, you can basically apply any board of any shape and convert it to a straight edge, without looking at it. Though in cabinet work there can be many reasons to size up the board very carefully and make best use of the material at hand.