First, I know release is in two days and this stands exactly zero chance of making it into the release if not already addressed.
Second, I acknowledge that maybe I just don't understand what's going on but, if that's the case, it indicates ambiguity in how the XP data is presented to the user.
That said, the "Experience Gained" value on the Post Match summary screen is meaningless. Either I don't understand what's going on as a result of poor presentation or the calculations are simply incorrect. It seems that the sum of the starting XP displayed on the XP progress bar and the XP displayed under "Experience Gained" should equal the XP displayed in the progress bar after all the calculations are finished. That is not the case, though.
Here is an example, recorded today, of what I mean: