为什么nwkTransactionPersistenceTime和macPib.transactionPersistenceTime，_NIB.TransactionPersistenceTime各不相同？

我用函数NLME_GetRequest读取nwkTransactionPersistenceTime这个项目的值，为什么既不等于macPib.transactionPersistenceTime又不等于NIB.TransactionPersistenceTime？

The maximum time (in superframe periods) that a transaction is stored by a coordinator and indicated in its beacon. This attribute reflects the value of the MAC PIB attribute macTransactionPersistenceTime(see [B1]) and any changes made by the higher layer will be reflected in the MAC PIB attribute value as well.(摘自ZigBee Specification 2010)

nwkTransactionPersistenceTime项目值默认是0x1f4

nwkTransactionPersistenceTime项目应该怎么读写？我怎么读出来等于0？