Does the percentage increase correctly?
It very well may be (but I don't know for sure) that the visual presentation for each step has a minimum of X pixels, since the purpose of the progress bar is to let the user know things are still running in the background, so usually a 1:1 visual representation of the current point of the range is not important.
You can test it by having a progress bar in a test application, where you connect lets say a slider to it.
There you can see exactly the "leaps" it will take not dependent of events.