Please explain initial audit sample size vs. sample size for confirming accounts receivable. I've quoted the factors respectively below, however I do not understand why the factors are different.

"Four factors determine the INITIAL sample size for audit sampling: population size, tolerable exception rate (TER) , acceptable risk of overreliance (ARO), and estimated population exception rate (EPER) . Population size is not a significant factor and typically can be ignored, especially for large populations (p. 494)."

versus

"The major factors affecting sample size for confirming accounts receivable fall into several categories and include the following:

Performance materiality

Inherent risk (relative size of total accounts receivable, number of accounts, prior-year results, and expected misstatements)

Control risk

Achieved detection risk from other substantive tests (extent and results of substantive tests of transactions, substantive analytical procedures, and other tests of details)

Type of confirmation (negatives normally require a larger sample size)" (p. 540).