How exactly are chapters founded?

[Bryan Blaire]

The "unknown origins" could also occur by not always having the predecessor Chapter submit the training cadre. The training cadre themselves may not even question things, because it's not like they are likely to contribute much to the gene-seed pool either, and anything that doesn't work quite right may just be considered "an anomaly" and disregarded. After a while the records may not match with other things, "they don't fight like other Ultramarines decendents and they get so angry", "they can still spit acid", "their skin isn't as white as ours", and everyone likely just throws their hands up in the air, because good luck getting answers out of the Adeptus Mechanicus or Terra/the Administratum. It doesn't have to be nefarious or even purposeful.