Probably due to rounding errors, the map tiles on the 3rd and 4th zoom level are not 100% accurate. See the bottom right area of the map and zoom in/out to see the issue. This should be fixed in the java app that generates the tiles.
Small update on this, I checked the Java program but the issue does not seem to be a rounding issue of sorts. It outputs the correct coordinates/sizes +/- 1px, so it should work but it still doesn't. Further investigation is required.