It seems from Tosfot D"H "Ee Kalish" that although the Gezera about Suka according to Rava (from Prasmakya) is because maybe there will be more than 20 Amot and he will not notice (same as in Rashi), however they (the Tosfot) don't learn the same Gezera in words of Ravina (Suka Deorayta) and learn that the Gezera is because of "Ee Kalish". I didn't understand what is exactly the Gezera, Sechach inside 20 and above 20 Atu what? Also what make Tosfot understand that the Gezera according to the two Emora'im is different?
(Note that Deorayta/Derabanan and single/multiple people is just a reason why the Gezera is not in bnoth Suka and Kora but doesn't explain the Gezera itself.)
According to the way I understood Tosfos, Tosfos' intention is as follows: According to Rava who holds of the Gezerah with regard to Sukah (to invalidate it when part of the Sechach is above 20 Amos) since a Sukah is used primarily by a single person, if the twigs reach down below 20 Amos by a considerable amount, even a single person will pay attention and notice. In such a case Rava would not make a Gezeirah. However, according to Ravina who holds that the Gezeirah of Sukah is dependent on the fact that it is d'Oraisa, even if there is a Heker for the person dwelling in the Sukah nevertheless the Gezeirah still applies since it is appropriate to make a Gezeirah for a law that is d'Oraisa.
Tosfos answers that even for something which is d'Oraisa, such as Sukah, we do not make a Gezeirah when there is no fear at all that the wind will blow the branches away.
By the way, note that the TOSFOS HA'ROSH asks the question according to both Rava and Ravina, as you suggested. Perhaps he understands the Gemara the way you explained, that the question applies according to both Amora'im.