an A in a college prep course counted for 4 points toward your GPA, but an A in an AP or Honors course counted 5 points.
This is a fairly recent development. Students were figuring out that if they got a B in an AP course they were bringing their grades down so they elected to go the regular route and get the A's instead and keep their grades up.
Some schools the scale for AP is not based on the A+ but all on the scale. An "F" is still 0 points but a "D" is 2 instead of 1 and an "A" is 5 instead of 4. C's and B's scale respectively as well.
This rewards the kids for actually qualifying for an AP class instead of punishing them if they take an AP class but don't score quite as high as if they would in a normal class.