— - I noticed that before when I was making a .capx to help in the forum. But I believe that is not a bug because the Window Width and Window Height is based on the browser's size or screen size(if fullscreen). You should use "OriginalWindowWidth" & "OriginalWindowHeight" for that to have an accurate measurement.
Edit: I also found something that worsen the inaccuracy with the positioning of your test-sprite. I noticed when I replaced the "WindowWidth" and "WindowHeight" with "OriginalWindowWidth" & "OriginalWindowHeight" that the position of the sprites are still misplaced, that's when I noticed that the image size is (5,5) and when I set the image point to middle the image point goes to (X:2.5,Y:2.5) but when I close the image editor and reopen, I saw that the location came to (X:3,Y:3) and it looks like it rounded it off and it will make problems with positioning. I don't know if the image points inaccuracy is a bug but for the moment I suggest using TiledBackgrounds or resize the image of the sprites to numbers that can be divided and still retain a whole number.