1) partly due to smaller size (design choice, not better engineering), but also more efficient components from AC inlet to the drive wheels, which is better engineering
2) clearly better engineering
3) design choice, but also better human factors engineering (Tesla has a better user interface overall, by far, but not this one item)
4) clearly better engineering
5) design choice
6) pricing choice (GM can lose money if they choose; Tesla needs to fund future development)
7) design choice, but perhaps another example of better human factors engineering (if you like the green ball)
8) design choice (and a much better one I think)
I will add a few more examples of better engineering on the Volt:
1) Charge based on departure time option (better human factors engineering)
2) Quick and easy lift pedal regen selection, by selecting D or L, or steering wheel paddles in the 2016 (ditto)
3) Best Quality Relibiity and Durability of any three year old car in its size class (class C). This take lots of engineering prowess and hard work for design, manufacturing, and supplier quality engineers.
Of course, Tesla has done better engineering in many areas. That is another list. In my opinion, the Volt, the Model S, and the Model X are by far the best cars on sale today, at any price.