How is the link#xx generated and does it have to be the same in HA setup?

[someusername]

We have a HA setup with a bunch of VLANs. I was comparing the routing tables of both FWs for unrelated reason and saw that the Gateway link#ID was the same for some networks and different for others.

Is this a problem? How is this ID generated/controlled?

On another pair of FWs, all link# are the same.

Here is an edited excerpt of the routing tables:

Thanks in advance!