Ah! - That makes sense.

Guess I hadn't noticed because I'd forgotten that (similar to the test you proposed) after originally setting my card's display rez, it just seemed natural to adjust the monitor's H & V Size until true circles looked round and a square did in fact physically measure equilateral again.

Although this does yield slightly wider borders at the top & bottom than at the sides, I had (presumptuously) just assumed everyone did something similar to calibrate their picture tube, regardless of aspect ratio, rather than just filling the screen side to side and top to bottom. I now understand your point though and see the difference. Thanks for the elaboration FN.