Its the internet, so you never know how truthful everyone is anyways. I am sure that people have lied about their scores on here. Besides if someone really got a 400 I dont think they would be posting on a GMAT board about that.
It appears to me that the majority of this board is made up of people who are planning on attending fulltime MBA programs and are take the GMAT very seriously. Someone who spends time on here also probably spent or will spend a good deal of time studying for the GMAT...thus they will score higher than someone who is going to do a local parttime program paid for by their employer and will just go in and take the test to get into. I know if I wanted to go to one of the local colleges that offer MBAs and have a 550 average for the GMAT I wouldnt bother studying. I would go to the test center and not really worry to much about it. Aiming at a highly rated school requires a good GMAT score so I wouldn't want to risk the test taking fee and having a bad score on my record...so I am spending several months preparing before even thinking about taking it.