Here is the official blog post regarding skill groups:
http://blog.counter-strike.net/index.php/2012/10/5565/
The only other official information we got is this comment from a Valve employee on Reddit (highlighted in yellow):
https://www.reddit.com/r/GlobalOffensive/comments/2g3r4c/the_ultimate_guide_to_csgo_ranking/ckfhfir

Originally posted by "vitaliy_valve":

When competitive matchmaking as we know it now was introduced in CS:GO in late 2012 we switched all non-competitive game modes to use simple ping-based matchmaking. For Competitive, we built a CS:GO-specific competitive ranking system that is significantly different and more complex than Elo.

The CS:GO competitive ranking system started with ideas based on Glicko-2 rating model and improved over time to better fit the CS:GO player base. All computations are performed on our matchmaking backend and multiple matchmaking parameters describing scientific set of rating variables of a player are represented to players as a their Skill Group. You should be able to find papers on rating systems involving rating volatility and rating deviations online to get a better idea about why our complex competitive matchmaking parameters cannot be represented as a single numeric value.

Anything else, for example that "guide" (read: assumptions) which the Valve employee commented on, is just speculation from random users who don't know any more than you do.
Nobody knows what exactly affects your skill group, or by how much. Just play your best all the time and lead your team to victory. And most of all, don't worry about your skill group icon too much and just enjoy the game.