the "official" number is based upon the Lemon study in 1980 in which he based it on nitrogen balance. He found at a certain level of protein being eaten depending on activity level, it will start being wasted by the body and literally be pissed out...
However, nitrogen balance != protein absorption. Whats been discovered in further studies is that while at a certain level of protein intake you will start wasting it, you will not however waste all of it. It's a case of diminishing returns the higher you go past the nitrogen balance point is all..
So lets say your body hits the nitrogen balance point at 100g. since you're at balance, all 100g is absorbed. Now lets say you decide to eat 150g. You will measure something like 25g of that being wasted and just pissed out of the body, but what will happen is 125g will be absorbed. (number not accurate, just illustrating what happens) So even though you're above the protein RDA, you're still getting a net benefit by doing so.
For me, consider the protein RDA as your starting point. You should never go below that amount. And I consider the 1g/lbs lean body mass a general optimal level to shoot for if you're active.