They don't overstate what they get, only report numbers they actually get with their test barrels. Of course the manufacturers test barrels are usually better quality with tighter chambers and more precise tolerences in the barrels than the average hunting barrel. Some rifles will come very close to advertised velocities, others not. I've seen guns with equal length barrels have more than 100 fps difference with ammo fired from the same box. I've seen a 22" gun shoot the same ammo faster than another 24" gun. This is why it is necessary to shoot the ammo you are going to be using over a chronograph so you know how fast it is in your gun.
Same with BC. They don't misrepresent them. There are different ways to measure it. Bergers method is probably more accurate and they list 2 different BC's on their box, measured 2 different ways so buyers can have comparable numbers to compare with other brands.
BC also changes as velocity changes. A bullet with a listed BC of .475 might be .475 at 3000 fps, but as the bullet slows down at longer range the BC also changes. The online programs used to calculate trajectory and energy don't compensate for this and often show faster bullet speed at long range than you will actually get. Those are fine for getting an idea of what to expect, but there is no subsitute for actually shooting at those ranges to know for sure where you are going to be hitting.