Its not uncommon for a lumber company to cheat you on scaling wood. I stopped using a company that started shorting me more than 20% when the company changed hands. To be fair though a lot depends on the policy of the company. The lumber is scaled at the saw mill when it is cut from the log and they have a choice of selling the wood as it was scaled green or scale it after its kiln dried and marking the price up to account for the shrinkage. When they scale lumber they don't use tape measures, they use a lumber scale. Depending on the length of the board it may only measure about 3/4" intervals and just reads in board feet. They measure at the widest part of the board and if its between numbers its rounded up to the next board foot. It may be if you used a lumber scale and rounded the boards up to the nearest board ft. and allow for shrinkage the 21' you got started out as 26' when it was green. Also if the board was straight line ripped, you also paid for the strip they trimmed off. The amount of shrinkage should have been closer to 7% though and they don't discount for defects.