PDCPRX_HFN的问题

36.323 的5.1.2.1.2中描述了一个PDCD PDU落在的重排序窗口（reordering_window）之外的处理方法：

For DRBs mapped on RLC AM, at reception of a PDCP Data PDU from lower layers, the UE shall:

-     if received PDCP SN – Last_Submitted_PDCP_RX_SN > Reordering_Window or 0 <= Last_Submitted_PDCP_RX_SN – received PDCP SN < Reordering_Window:

-     if received PDCP SN > Next_PDCP_RX_SN:

-     decipher the PDCP PDU as specified in the subclause 5.6, using COUNT based on RX_HFN - 1 and the received PDCP SN;

我的问题是，上述红色部分为什么是RX_HFN - 1,而不是RX_HFN + 1， 即收到的PDCP sn是否可以落到下一个MAX_PDCP_SN周期？