A recent annual report for Ford Motor Company contained the following note:
Significant Accounting Policies
Depreciation and Amortization of Property, Plant, and Equipment
Property and equipment are stated at cost and depreciated primarily using the straight-line method over the estimated useful life of the asset. Special tools placed in service before January 1, 1999 are amortized using an accelerated method over the estimated life of those tools. Special tools placed in service beginning in 1999 are amortized using the units-of-production method. Maintenance, repairs, and rearrangement costs are expensed as incurred.

Required:
Why do you think the company changed its depreciation method for special tools acquired in 1999 and subsequent years?