Varies between my two Kongu and Hahli as well.
A set number of distinct patterns make sense; it’s the same thing commonly noted with the poisoned Hau Nuva variations. I’d recommend the explaination provided in this Eljay Johnsen show episode (timestamped).
This initially talks about Krana, which I have included for context, but it does move onto discussing multi-color parts.
(Note: This does not appear to hold true for the 2007 Sea Squids. It is unclear why.)
I’d imagine the differing side of the injection mark is due to the different positions in the mold. Afaik, all of these parts only have on element ID, implying one mold each.