Yep you are getting a roll off effect at the edge, really noticeable on the right slab making a convex surface, and that is due to wobble when you try to sand blocks standing on their edge. Try a sanding set up where the blocks lie flat and the sandpaper is held 90 degrees to flat. Remember you can work on one slab at a time, they don't need to be exactly the same size, if that works better for you, I always do one slab at a time. Also, if you route the edges carefully using a very straight guide, you should not have to sand at all, just a quick scrape being careful not to round over the edges and off you go. It's worth sticking at it, it is very satisfying to get a perfect joint, why not get some cheap wood and practice your technique first. Good luck with it.