I think there might be a slight misunderstanding of the requirement. According to the Talmud (Menachot 33a), a mezuzah that is hung like a bolt is pasul (improper). In other words, the mezuzah should not be hung like a bolt.
There is then, as you mention, a disagreement between Rashi and Rabeinu Tam about what a bolt looks like. Rashi says that a bolt is horizontal and the mezuzah should be vertical. Rabeinu Tam says the reverse is true - a bolt is vertical and the mezuzah should be horizontal.
Medieval Ashkenazi codifiers like the Tur and the Rema tried to satisfy both opinions by requiring a mezuzah be affixed in a way that is neither vertical nor horizontal, so that it would not be like a bolt according to either position. Since the disagreement boils down to which position to avoid, Ashkenazim err on the side of avoiding both. Because the disagreement was on what to avoid, by taking a middle position these Ashkenazi poskim are trying to be 100% correct according to both positions, rather than a 50/50 proposition.
It is worth noting is that the Shulchan Aruch rules differently. Yosef Karo takes Rashi's position to the exclusion of Rabeinu Tam's and rules that a mezuzah should be vertical. That is what Sephardim do today.
A good summary of all this is also provided here and here