Exp. % is the Pythagorean Win % for your team. It has nothing to do with how many games you can expect to win going forward (it tends to be a slightly better predictor than your actual winning %, but not greatly so after 50 or 60 games), and it's based on run ratios rather than differential.
Exp % = (runs scored)^2/[(run scored)^2+(runs allowed)^2]
= 1/[1+(runs allowed/runs scored)^2]
If you assume that baseball scores follow a Weibull distribution then this formula statistically predicts how much a team should win based on its runs scored and allowed. However, teams with bullpens that are significantly superior or inferior to their starting staffs may perform above or below their overall anticipated win % in close games, skewing their real win% from their Pythagorean win %. The fact that not all pitchers on a staff are equal does interfere with the formula, which assumes that runs scored and allowed are independent; in reality, the closeness of a game can impact which pitchers are used, and not all pitchers on a team are equal.