Why Epoch time is required for RM126x p2p in US?

FCC Radio regulatory standard (applied to RM1262) is 47 CFR FCC Part 15.247 FCC CFR FCC -

https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A/part-15#15.247 -

We declared LoRa 125kHz as FHSS, so the Epoch Period = 0.4 x N where N minimum is 50, for RM1262(LoRa 125kHz) we used N=64.  

For more information, refer to  FR2D2902_RM1262_FCC 15.247 LoRa_902.3 ~ 914.9MHz (FHSS_125k).pdf in https://www.lairdconnect.com/documentation/fcc-certifications-rm1262