Sure. But my particular monitor is "smart" enough to left-justify on the start of the visible area. Initially when I tested it was more centered. I strongly suspect that the monitor firmware tries to detect where the visible signal starts (because different VGA cards might be out by a few nanonseconds one way or another) and then remembers something like "oh this card has a back porch of 2.1 uS, so we'll start drawing after that each time".
Judging by the logic analyzer output the signal is fairly correctly centered.
The monitor has a "horizontal position" menu setting. If I play with that I can center it. Ditto for vertical.