the before picture is always a really badly taken photo.. the person has no make up on, is dressed in a very flattering clothing, and always has a depressed facial expression

in the after shot its a professionally taken shot, the person is wearing trendy clothing, has very white teeth (digitally enhanced) and is smiling

so even if there isnt much of a difference, it makes it look like there is
case in point

http://www.childrenfirst.nhs.uk/teens/images/features/michelle_before-after.jpg

im not denying theres no difference.. but i think they are exaggerating it