# +0

There are *a lot* of different components to a probability distribution, and even more components when we compare two different probabilitilistic functions that output qualitatively similar things but with different ranges in addition to different distributions.  By adding a +0 modifier, you keep your 1d20+X from having a different range than your Nd20 and thus make them far more similar mathematically than if you add any larger value.  

Obviously, this isn't the best way to make the *average* values similar, but average value isn't nearly as important as range in typifying a roll for RPG purposes-- the minimum value you can roll determines what you can always do all the time with no chance of failure, while the maximum value determines what you can do when not under time pressure, at least in typical dice-based long-form-campaign GM-led RPGs. The average roll matters a lot, too, of course, but it's not *as* important and here we have to pick which we care about.  

Since we are adding the same static modifier to all rolls and that just moves the distribution around, we can really do anything about the other more-important-than-the-average aspects of the distribution, like how Nd20 take highest has a much smaller standard deviation or isn't a uniform distribution.