Why are my ratings so sloppy?

I am currently 682 on blitz, but I win just about half my games.

Why?

Youre a beginner, playing 10 minute games.  

I heard that everyone starts with 1200, so shouldn't I be about 1500 by now????

Do 10-minute games have a disadvantage?

people usually start around 400- 600 strength once they learn the rules of chess, and then based on the ELO system you will approximately DOUBLE your strength and knowledge every 130 pts or so. so this means to gain 500 points you will have to get 8x stronger or learn 8x more than you already know . for us non-masters tactics (calculation training) and slow games, analyzing them afterwards is the most effective way to get stronger . once you get your tactics to 1400+ I can recommend some books or study material

and logically if you are winning 50/50 with equally rated opponents you won't change rating . if you win more than 50% or win 50% vs higher rated players that will do the trick

Post number 7 is right, and the computer generating the opponent for you is designed to select someone close to your ability: therefore everyone from GM to beginner should win about half the time (because better players are playing equally tough competition). If someone is not winning 50/50 but say 80/20 (arbitrary figure) then they are either: 

1) not having the computer generate the opponent by either sending "challenges" personally, or changing the computer generating deviations via the settings

2)They are "inaccurately" rated as they are improving a ton (or dropping a ton), but more likely they have a "new account" as the rating has not adjusted to the "true" rating yet.

3)The rare, but obvious, they are cheating/sandbagging

4) Any other reason plausible that I left out

There is no difference among various time controls (but some are separately rated such as daily versus blitz). To improve simply win more often than you lose, although this is clearly easier said than done.