Why not 3/5ths? (Shabbos 34b)

Rabba says that shkia is three parts of a mil and the gemara tries to figure out what it means. Can't be 3 halves or 3/3rds because then he would have phrased it differently so he must mean 3/4ths. That's fine but then the gemara questions what Rav Yosef means when he says 2 parts. Can't be 2/2 because then he would have just said 1. Can't be 2/4ths because then he would have said a half so it must be 2/3rds. Why by Rav Yosef did the gemara entertain the possibility that he meant 2/4ths but by Rabba the gemara didn't even entertain the option of 3/5ths?

I was happy to see the Ritva asked this question but his answer didn't satisfy me so I was even happier to see the Rashash. The Ritva says that the gemara will start with the lowest until it gets to a number that makes sense. So in Rabba it never gets to 3/5ths. It really shouldn't have gotten to 2/4ths in Rav Yosef either but it thinks it's a possibility since we already established that Rabba was talking about fourths. The Rashash asks if that's the reason that the gemara had the hava amina that Rav Yosef meant fourths then why did we fall off that answer? The gemara only rejected the answer because if Rav Yosef wanted to say two fourths then he would have said one half instead. But maybe he said two fourths only because Rabba was already talking in fourths?