This is a choice I made at the beginning about how to handle bevels between faces that do not meet at 90 degrees. The way Blender does this is to always make a profile that is a quarter of a circle (or quarter of a superellipse, if the profile parameter is not 0.5). Then if the faces don’t meet at a 90 degree angle, a transformation is applied that takes the 90 degree angle into the actual angle. This gives the effect you note in your diagram.
The alternative you show looks easy but is actually not so easy. What you are doing is inscribing a circle in the angle and noting where the circle is tangent to the two enclosing edges, and then using an arc that is that percentage of a circle rather than a quarter circle. The problem is: what if the tangent points don’t match where the bevel wdith parameter says they should be? With circular profile and a non-percentage width type, it will work out ok. Admittedly this is a very common case, but what to do in other cases?
I suppose we could add an option to try that way. Do other users feel this is an important option to have?