This is incorrect.
When only width and height are provided it resizes the window as specified and then centers it on the screen. This stems naturally from its use when the window opens, if you only specify width and height it needs to put it somewhere.
You can base it on the current position of the window using:
window.KeyboardMaestro.ResizeWindow(window.screenLeft+","+(window.screenTop+19)+",220,100")
19 is the current title bar height, since ResizeWindow actually expects the content size rather than the window size.
I will add the context sensitive WINDOW function for future versions (which will also return the content size in this case for compatibility with the behaviour in this instance).