I have one of the RS 60-100X microscopes too. I use it almost always at the 100X magnification. I can see the variations in scratch paterns usually, and more clearly at that setting. So, it's probably the limitation of it.

I think because I've been working a that mag level, I've gotten used to it, and have learned to interpret it. Looking at the same edge at a different level of magnification would probably give me a different impression of its state, especially if it was the only indicator that I was using to evaluate.