I'm not going to try to explain it anymore, from annual report 2010:
Accounting Quantity The accounting quantity is our estimate of the quantity of airplanes that will be produced for delivery under existing and anticipated contracts. The determination of the accounting quantity is limited by the ability to make reasonably dependable estimates of the revenue and cost of existing and anticipated contracts. It is a key determinant of the gross margins we recognize on sales of individual airplanes throughout a program’s life. Estimation of each program’s accounting quantity takes into account several factors that are indicative of the demand for that program, including firm orders, letters of intent from prospective customers and market studies. We review our program accounting quantities quarterly.
Commercial aircraft production costs include a significant amount of infrastructure costs, a portion of which does not vary with production rates. As the amount of time needed to produce the accounting quantity increases, the average cost of the accounting quantity also increases as these infrastructure costs are included in the total cost estimates. This has the effect of decreasing the gross margin and related earnings provided other factors do not change.
The accounting quantity for each program may include units that have been delivered, undelivered units under contract, and units anticipated to be under contract in the reasonable future (anticipated orders). In developing total program estimates, all of these items within the accounting quantity must be considered.
