The difference in these decorations are the result of the many different manufacturers that made them. Each one made their own molds and thus the differences. Over the years molds are worn out and needed to be replaced, so even from the same munufacturer they can show some differences. It also happened that one manufacturer bought the other one and got a hold on their molds so that two different manufacturers can have the same design. (Still following? 😛)
Common manufacturers are/were: Wolfers, P. Degreef, Mauquoy, Fisch, Fonson, Walravens ... and many others. The earlier ones were made by jewelers like J.G. Dutalis, Allard, C.J. Buls, J. Heremans, etc. Some of them are easy to recognize by their design.
If you like these differences the work of Antti Ruokones, who's mentioned a few post back, is a must. He compares many makers and their differences in design. It's probably not 100% accurate (as it's nearly impossible) but it's the best attempt to list the differences and linking them to a manufacturer that I've seen this far.
Not necessarily, the way they are stored has a big influence on how they aged. If they are always kept in the box, locked in a drawer and never came in contact with the sun, they will look far better than one that is mounted in a frame that hangs on a wall and comes in direct contact with light. Also is the medal worn a lot or not? Is it cleaned in the past or not? ... (Sun)light and rubbing are a medal's worst nightmare.