Not only are advertised velocities grossly overstated (and sometimes understated)...advertised BC's are too. Many are off by as much as 10%, most are off around 4-5%.
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA554683
Berger bullets are an exception...Berger thoroughly tests theirs, but not everybody has Litz as their Chief Ballistician.