@tauscher that explains it!
I have added it to the OSArch wiki guides - a new page for ArchiCAD setup things to consider for OpenBIM: https://wiki.osarch.org/index.php?title=ArchiCAD_setup
In my opinion, I think a better solution would be for ArchiCAD to just randomly generate it, but then allow a field to override it in the UI if you want it to match a specific value … the use of such a seed decreases randomness.