A.D.M. or Additional Dealer Markup was a gift from our previous Gov. Ev Mecham who used it first on some spiffy Pontiacs back when he was only small time (a car dealer).
Although eventually you can purchase most vehicles below the M.S.R.P. (Manufacturer's Suggested Retail Price), it is still only the manufacturer's suggestion. Ev's idea, which is still used today, was based on simple supply and demand. When demand exceeds supply, price goes up.
For a new vehicle model, and especially a 'desireable' one - such as the Sequoia, or for Ev in 1984 the MSE Edition Firebird, there is a brief period where the demand is so much greater than the supply that the dealer can and WILL sell the vehicle for MORE than the factory suggests.
The key here is that NO ONE HAS TO PAY A.D.M. - but if you want that new model NOW, you may have to pay more for it. Next year, when there are more out there, when the initial craze for the new model settles down, well, then you probably could negotiate yourself a deal below M.S.R.P.
However, if you are one of those that just can't wait, pay the A.D.M. and brag to your friends how much more you had to pay just to be the first on your block with that shiny new buggy.