When I think about differences between the novels and the comics - and there are many of them - I always keep in mind Jake Chambers’ famous phrase, “there are other worlds than these.” The Dark Tower contains many levels, and within those levels are parallel worlds which mirror each other, but which are not exactly alike.
I always view the Dark Tower comics as existing in one of these parallel worlds. If the Dark Tower novels exist in Tower keystone, or the central world of the Dark Tower universe, then the Dark Tower comics exist in a spinoff world, one which is very similar to, but not exactly the same as, the one where The Gunslinger, The Drawing of the Three, The Waste Lands, Wizard and Glass, and the rest of the Dark Tower novels take place. When you think about it, this idea of parallel worlds isn’t so very different from contemporary theories in quantum physics. According to these theories, every time we make a decision and follow a certain path in life, another reality is born where we make a different decision and follow a different life path. For example, imagine that you are walking down the street. You come to a crossroad. You have a decision to make - you can either turn left and go home or you can turn right and go to the shop and buy a lottery ticket. You tell yourself that buying a lottery ticket is stupid, so you go home and take a nap. However in a parallel reality, a different of you turned right and bought a lottery ticket…and won! (But don’t be too sad that in this where and when you didn’t buy that lottery ticket. On another level of the Tower, you were hit by a van before you even reached the store.)"
— Robin Furth