Question: Efficiency Standards. The U.S. Postal Service (USPS) advertises prompt delivery schedules for express mail (overnight delivery) and priority mail (two-to-three-day delivery). The USPS knows various risks that may arise to thwart a timely (as advertised) delivery but believes that systems and controls are in place and operating to mitigate the risks. The USPS advertised that 94 percent of express mail and 87 percent of priority mail was delivered on time from the time the mail was postmarked to the time it reached the destination post office. However, a consulting firm studied the USPS operations and determined that the express mail arrived at the recipients' addresses on time 81 percent of the time (not 94 percent) and the priority mail arrived timely 75 percent of the time (not 87 percent).

Required: What can account for the difference in these performance statistics between the USPS delivery rates and the consultant's rates?