I am guessing based on this:
https://github.com/kovidgoyal/calibr...r.py#L706-L707
Probably done for the sake of the viewer, which technically converts a format into an OEB.

I realized that was a slightly awkward description (I am not sure you can actually convert them, also it seems to say "
view_kepub" so I am guessing no

) so I quickly reworded it. But you were quicker...