The reason that Sanheim "came out of nowhere" was that he was never touted at a top bantam-pick and hyped well ahead of his development.
In the 2011 WHL bantam draft, Fleury was a second round pick while Sanheim was a ninth round pick (this again shows the crapshoot nature of the bantam draft).
As a 15 year old, in 11/12, Fleury played 4 games and Sanheim played 0.
As a 16 year old, in 12/13, Fleury played 66 games and Sanheim played 0.
As a 17 year old, in 13/14, Fleury played 70 games and Sanheim played 67.
At the time of draft, Fleury had 140 games of WHL experience to Sanheim's 67.
THIS is the main reason he continually projected behind Fleury - scouts hadn't seen him as much. If you look at the developmental arc, Sanheim is a season behind. Considering he was about equal to Fleury at the draft, it's not a big surprise that he has shown exponential growth in his second WHL season.
The Hitmen have always been a longer developmental arc team than other WHL teams. With the rare exception of top talent like Bean or Virtanen (1st overall in the bantam draft), players rarely come in as a 16 year old and play the whole season.
However, Calgary consistently puts together a better team than Red Deer. Which means that Sanheim is typically going to ice a better team than Red Deer and there may be a teammate effect going on between Sanheim and Fleury. Thus, let's contrast another WHL defencemen in the same draft class: Ben Thomas.
[Elite Prospects claims he played 0 games as a 16 year old, but I distinctly remember seeing him in the 12/13 season and being very impressed. I would estimate he played about 5-10 games in that season.]
Thomas was drafted in the 3rd round of the same 2011 bantam draft, played a few games as a 16 year old, and was initially ranked higher than Sanheim for the 2014 NHL draft. In fact, for the 2014 NHL/CHL top prospects game, Ben Thomas was given the "home-town" spot when the team was named in mid-November (Fleury also played in this game, while Sanheim did not). By the time the game rolled around in mid-January, it was clear that Thomas was out of his league and Sanheim was an oversight.
This is Sanheim's second career WHL goal:
http://www.hitmenhockey.com/video/35241
from just after Christmas last season (January 2, 2014). His footwork over the Christmas break improved astronomically, and it's paying dividends now as his best attribute is his ability to move around while making plays, or being mobile to create plays. It was as though the rest of the team sat and ate over the holidays and he just went and learned to skate. From that goal onwards, it was all about moving up the draft rankings for Sanheim.
I don't know how it will translate to the NHL level, but he is absolutely great moving laterally at the blueline with the puck -
both left to right AND right to left. I've never seen a defenceman, even at the WHL level, be able to do it both ways like this before.
As the play that was GIF'ed above (too far back to quote) - it shows how great his mobility is. He's never making the play standing still, and always creating options by moving around. It also shows his passing, which is always phenomenal. I know I'm biased, but I think there is a very high probability that Sanheim is the second best defenceman to come out of the 2014 draft (hard to argue against Ekblad).