Using all teams from 2010-14, the relationship between OPS differential and winning pct is
Pct = .5 + 1.3246*OPSDIFF
The Cubs currently have an OPS differential of .175. So they should have a winning pct of about .732 instead of "only" .662. They should have 56 wins, not just 51. Baseball Reference (where I got all my data) shows a Pythagorean total for the Cubs of 56.
Bad timing might be the reason the Cubs have not reached 56 wins.
The table shows some of their hitting splits.
|Late & Close||0.205||0.329||0.324||0.653|
The table shows some of their pitching splits.
|Late & Close||0.198||0.286||0.303||0.589|
The hitters do well with runners on and w/RISP but overall they do better in Low Leverage situations and, of course, late & close is a weakness (which is just a random thing). The only problem for the pitchers seems to be a somewhat high OPS w/RISP but they do much better in Medium and High Leverage Cases.