It's a known issue with the C3 grendel layer that they will lay eggs from any genome that starts with 'G'. If a norn genome starts with 'G', it might be randomly hatched as well. I can remember having a world where that happened. It kept picking the norn genomes by accident, so I had several extra norns, before it finally picked a few grendel genomes and stopped laying eggs. Or was that the Ettin layers & the letter 'E'?
Both, that's why it's always best to start your genome names with "norn", "gren" or"ettn".