We are not perfect in our methods.
An ideal edge might be perfectly keen. Sadly we will rock a bit as we sharpen. at the very end we tend to rock just a smidgeon, and cause a fine dub.
Now we have a bit of an issue.
When we rock it back flat, the end of the blade no longer contacts the surface. When you sharpen and sharpen and it seems to have no effect. This is probably the cause.
It is mostly possible to avoid a double dub, since the flat of the blade can be kept fairly flat. A double dub, will definitely raise the angle of the blade, making it less ideal.
The problem is, that once dubbed, you no longer have contact with the edge as you sharpen, unless you either increase the angle on purpose, or grind away enough steel to again have an ideal surface. Without contacting the edge, no sharpening is going to happen. This is where stropping comes in handy.
With fine grit in and on a slightly flexible surface, contact with the edge can still be made.
This solves a lot of human error, and allows us mortals to sharpen well by hand. Stropping is not perfect, it does dub a bit.
Below is what happens when you strop with a lot of pressure.
You get a more extreme dubbing. This means you have lost some of your potential sharpness.