I think your question can be answered with the help of a "Diyuk" in the words of RASHI 3b DH BE'CHATZI. Rashi writes that we learned a Halachah to Moshe from Sinai that Tzroros is Mamon and not Kenas. ROSH in fact disagrees with this and writes that one does not require the Halachah from Sinai to teach that Tzroros is Mamon, but rather the Halachah is necessary to teach that it only pays half Nezek. However CHESHEK SHLOMOH (printed after MAHARSHA on p. 80) writes that Rashi in contrast maintains that the fact that Tzroros only pays half is derived from logic, not from the Halachah from Sinai, whilst the Halachah teaches that Tzroros is Mamon.

It seems that the reason Rashi learns that logically Tzroros only pays half is because the animal does not damage directly but only through what it's feet kick up. Therefore Tzroros is not the same sort of Derech Hiluchah as Regel because the latter is direct damage by the body. This then is Rashi's "Havah Amina" that it should be similar to Keren - just as Keren is not full damage because it is unusual, so too Tzroros is not full damage because it is indirect.

Therefore it is only because of the Halachah to Moshe that Tzroros is Mamon, that we know it should be compared to Regel, which is also Mamon, and not compared to Keren which is Kenas. Once we know that it is not similar to Keren, Rashi writes that it comes "mi'Ko'ach Regel" - that it is similar to Regel because both Tzroros and Regel are damages done by the animal's moving. However, even at the end of the day it is not totally similar to Regel because since the damage is indirect, it logically should only be half Nezek.

D. Bloom