That's because you are pushing up a burr towards the front. Virtually all sharpening is based on removing material until you raise a burr and then moving that burr side to side until it breaks off. Then you hone what's left. I sharpen beauty scissors, knives, and tools for a living, and it's largely the same with most any tool.
Some sharpening devices and techniques will fool people because that burr cuts on the forward stroke(in your example). But, it's a weak edge and will break off as it hits something hard.
We kid ourselves if we think that something that "catches on you nail" is the best cutting tool. I will not argue that a file sharpened edge is better or worse than a glass-smooth cutting edge(have an opinion, but mixed results from 35+ years of bowhunting!). For the most part, both those methods have proven themselves to me. What doesn't work is assuming that a burr represents a super sharp cutting edge. It does not. If you can feel a definite burr on one side or the other, you are not done sharpening!