Seems to me like we either win games because of Demko or lose them because of him. He's either incredible, or pretty bad.
Obviously, these stats are going to be biased in favour of the results, but we can compare Demko vs Holtby.
First, the more standard case, Braden Hotlby:
In our wins, Hotlby is about an average goaltender, and in our losses, he plays poorly. Fairly standard I suspect.
Now, for contrast, see the extremes of Demko:
Note how insanely dominant Demko is in his three wins. Yet despite this, when he loses, he performs even worse than Holtby, despite having a higher overall average save percentage. Holtby seems to be much more consistent, but just not very good. Demko seems to have high highs, but also lower lows. A relatively small sample size also needs to be considered a weakness of this analysis, however.
All in all, it's probably fairly standard for young goaltenders, especially ones just starting to get more games. It reminds me very much of Markstrom in his earlier years, which is, really, not a bad thing. Just have to hope that like Markstrom, he eventually learns to be more stable and consistently good.