Expected Wins are determined using the teams' total points scored and total points allowed, and plugging those numbers into a formula very similar to the Pythagorean Theorem. The idea behind this formula is to come up with a better statistical measure of a teams' performance than simple wins and losses (or ties). You can read more about the formula here.
For the sake of comparison, I have included the results from the 2012 Chargers season. Also, I have not included the Chargers 2 playoff games from last season in this post.
Chargers Expected Wins vs. Chargers Actual Wins
Graph 1 shows the 2012 Chargers under Norv Turner, the 2013 Chargers under Mike McCoy, and the 2014 Chargers through 9 games. As you can see, the 2014 Chargers' Actual Wins of 5 matches up almost precisely with their Expected Wins of 5.02. This would seem to indicate the Chargers have performed just about as expected through 9 games this season. Therefore, the Chargers at 5-4 can't be considered either overachievement or underachievement.
Chargers Close Wins vs. Chargers Close Losses (i.e. 8 points or less).
Close wins are generally considered to be more attributable to random plays, and therefore should regress to the mean over time. As we can see in Graph 2, the Chargers have not been particularly lucky over the last 2 and 1/2 years, with a total combined record of 7-13. McCoy's record in close games currently stands at 5-8, which is partially attributable to his general conservatism on game days - in other words, his game management decisions haven't been giving the Chargers the best chance to win close games.
Chargers Solid Wins vs. Chargers Solid Losses (i.e. 9-16 points).
These games are a fairly solid middle ground between the randomness of close games, and the consistency of blowout games. In my opinion, these games illustrate the strength of a coaching staff during the week, preparing the game plan, and getting players prepared to play. Graph 3 shows the Chargers' record in Solid games the last 2 and 1/2 seasons, and shows that thus far, the Chargers coaching staff has done a better job devising a game plan and preparing their players to play than their opponents - or Norv Turner's staff - over that time.
Chargers Blowout Wins vs. Chargers Blowout Losses (i.e. 17 points or more).
Blowout Wins and Losses tend to be better indicators of overall team quality, as opposed to the ability of a coaching staff to prepare players during the week, or the ability of a coaching staff to make in game decisions which affect the outcome. Graph 4 shows the Chargers' record in Blowout games of the last 2 and 1/2 seasons. Although the Chargers have mildly improved since 2012, they're clearly not a dominant team, and there's plenty of opportunity for General Manager Tom Telesco and McCoy to improve the roster.
Chargers' Opponents Expected Wins versus Actual Wins.
Graph 5 shows the Chargers' 2014 schedule, with their opponent's current Actual Wins and Expected Wins. The Chargers' Expected Win total of 5.02 is represented by the broken yellow line across the graph. As you can see, with the exception of Arizona (which appears to be significantly overachieving this season), the Chargers have beaten every team they've played with an Expected Win total lower than theirs, and lost to every team with an Expected Win total above theirs. This does not bode well for the Chargers' 2014 playoff hopes, as continuing this pattern would lead to an 8-8 record.
Following Arizona, the next biggest overachievers the Chargers have faced is the Denver Broncos, who are currently .072 wins above the Expected Win Total. The other teams which have beaten the Chargers - the Kansas City Chiefs and the Miami Dolphins - are actually underperforming their Expected Win Total by more than 0.5 wins, meaning these teams are likely better than their records indicate.
Based on the data, the Chargers Actual Wins almost precisely matches their Expected Wins. Their 5-4 record is an accurate reflection of their performance. Unfortunately for them, they've already blown 2 huge opportunities in winnable games against roughly equal or stronger opponents.
In order to reach the playoffs, the Chargers will have to get more aggressive in games against tougher competition, as the Chargers will likely need at least 2 wins against teams with stronger Expected Win Totals, and games against such opponents are more likely to be close games where one play, or one coaching decision can make the difference.