Map Zooming changes depending on position

Upon using the new map zooming technique where the starting position stays in the top left corner, I attempted to find out where to start a map so that the center of the map is 0,0. Using calculation and the minecraft wiki, I found that starting the map at -1024,-1024 would do the trick or would be close enough. However, the zooming changed position on the last time and tried to put -1024,-1024 close to centered on the map. The first 5 pictures show each step of the zoom at -1024,-1024 and the other 4 are each zoom step at 0,0 (except for the second to last zoom). As shown, the zooming is inconstant. I've tried it a couple of times in different worlds and got the same results.