The Mishna says that if your Sukka is above 20 Amos, it's Pasul. R' Yehuda says it's Kosher. If it's not high ten T'fachim or doesn't have three walls or has more sun than shade, it's Pasul.
In Eiruvin it says that a Koreh (a beam that went horizontally across the opening of an alleyway, to serve as a distinction between the alley and the street, so you won't come to carry on Shabbos in the street just like you carry in the alley) that is above 20 Amos, you need to lower it. The Gemara asks, why by Sukka the Mishna says it's Pasul and by Eiruvin it tells us how to fix it.
Tosfos asks, why is the Gemara not bothered with the Mishna about Hadassim that has more berries than leaves? The Mishna there says it's Pasul and you can fix it by plucking off the berries. So there it says both it's Pasul and how to fix it.
Tosfos answers that the Gemara is only concerned that our Mishna should be parallel to the Mishna in Eiruvin, since it's basically the same Halacha of how high they could be. Therefore he asks why did our Mishna changed the way it was said in Eiruvin.
Another answer, that by Hadassim it needs to say that you can fix them. I would say, since it grew while Pasul, it can never be made Kosher. Therefore the Mishna needs to say that it could be fixed.
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