I blame the forces created by the gravitational pull ;-)
Fair enough, it seems the game only checks for tidal forces when determining when to display an oblate body and when a round one. Has anyone even seen different levels of oblateness either, btw? Seems to me they all look the same.
This suddenly became evident for me when I found yet another gas giant with very low orbit, and it was oblate.
The whole affair is rather sad, considering that most of the observed oblateness in reality is caused by rapid rotation.