What throws a lot of people off is the fact that average or above average pitchers get a disproportionate bulk of innings pitched. There are tons of below average or replacement pitchers who aren't good enough to get accumulate innings or shift the mean ERA, etc. that much. So even though an Edwin Jackson may have an average ERA+ and IP numbers of a #3 or 4 starter, he is certainly better than well more than half of pitchers who pitched in the MLB during the season. Mean vs. median. So fwiw, WAR may say that Jackson is "average", but there are still a lot more MLB pitchers behind him than in front of him.
Ex. Chris Capuano had a 1.8 WAR - which is "average". But he was also 39/374 of all NL pitchers in WAR.