This is only a useful measure for the case where you are aiming for DC10. If you are aiming for DC20 or DC 2, then the highest of 2d20 is more like a +1 bonus. DC 11 is exactly a +5 bonus and frankly, it shocks me that modern game designers can't fully grasp probability distributions and only focus on summary statistics.
I don't think they actually missed anything, but only used a simple measure. As [MENTION=78255]Truename[/MENTION] shows, if the original chance is 25-75% the number 4.5 is a good rule of thumb.
There are obviously other differences, but the chance to make a typical DC is important.