NIC device naming

I'm trying to understand how NIC device names are derived? I have 6 HPE DL380 G10s with identical NICs. There is an on-board 4 port 1Gb 331i. There is a 2 port 10Gb flexible LOM, 622FLR-SFP28 NIC and a 2 port 10Gb PCI-E 621SFP28 NIC. I installed all with a RHEL 7.6 iso image. I want to use the 621 NIC for the OS interface. On some of the servers, the interface was assigned the name ens1f0 and ens1f1. On others, it was assigned ens3f0 and ens3f1. Trying to understand the difference? Anyone have an answer?