Now cross-posted to Graphic Design SE, as per the discussion in chat.
In the question ‘What is the difference between the accent on q and the accent on semicolon?’, Joonas Ilmavirta suggested I ask a new question concerning the different design of the old and new accent. For some reason, they chose to design the tonos (modern Greek, right in the image) different from the oxia (Ancient Greek, left in the image). In most fonts, there is verily no difference at all, but in some (I would assume those with a longer tradition, such as Garamond), the difference is quite pronounced; as demonstrated in the post referenced, the Ancient Greek accents mirror each other, whilst the tonos has a steeper angle than the ancient oxia. As Ilmavirta commented, modern Greek is off-topic, unless in relation to Ancient Greek, and therefore my question is concerning the development from Ancient to modern Greek. Especially considering the transition from Ancient to modern Greek grammar and typography, why are these symbols different from each other? How did the oxia develop into the tonos?