You have not taken into account the Pixel Aspect Ratio - you have assumed the pixels are square, when for PAL they are not as it is grounded in CRT technology which used pixels wider than they are tall due to the way a CRT scans horizontally.

For example, PAL is 720x576 (4:3) with a PAR of 1.067

720/4*3 = 540

720/4*3 * 1.067 = 576.18 (Not sure about the .18, maybe I did the math wrong, but I am pretty sure this is your answer)