This happens frequently. I don't really know why, but I doubt is should be something to worry about. If you'll notice, the over-all image of the silhouette of the tortoise stays looking like it is supposed to. It might be that you are feeding a little too much, but the tortoise doesn't look fat. So, ???
My guess is that the other side will probably catch up in time. My male Roz came to me with a huge difference in the width of growth lines - more than 1/4 inch wider on one side.
I think a while back there was a similar thread, and someone suggested that the tortoise might have a favorite way to lie under the heat lamps, resulting in one side of the shell getting dried out more than the other. This would then affect how the shell grows. I don't think anything needs to be rubbed into this shell, since the scutes don't looks stuck. Just frequent soaking (2x per week for 20 mins) and daily misting the shell should do the trick.