“then clearly they are both kasher, so how can one of them be “wrong”?”
They are indeed kosher as we do not find anyone mentioning the shape of the branches are me’akov. See Rambam below (scroll thru all Halachos in that perek) – there is no mention at all about the shape.
http://hebrewbooks.org/rambam.aspx?rid=7695
The issue in concern is only the fact of how it really was in the mikdash.
Although, it is still hard to understand why the Rambam did not mention in sefer haYad how it was shaped, but in case if it would be posul the Rambam would have certainly mentioned it.