It is because 8 Highers is 8 x (number of UCAS points for each grade at Higher) whereas 4H + 4AH (same subjects) is 4 x (number of UCAS points for 4AH) as the UCAS points not counted for the 4 Highers as they are in the same subjects.
eg. 8 Highers at grade A = 8 x 80 = 640 UCAS points
4 Highers + 4 AH all at A = (4 x 80) + (4 x 130) - (4 x 80) = 520 UCAS points.
As for the subject of the thread, there would be no point on someone doing 6AH as they will never mean anything. They won't help you get a place at university anymore than 3AH and after university, your school grades count for nothing.