I think the day games are more of a factor in the teams keeping a consistent schedule. When you have a 1:20pm game, you are at the ballpark relatively early in the morning. When your games are at 7pm, you get to the ballpark in the early to mid afternoon. Most teams play 5-6 night games per week, plus a Sunday matinee, so their week-to-week "work" schedule is fairly consistent (traveling between timezones notwithstanding, as every team has to do that). So basically, most teams consistently work the 2nd shift, while the Cubs are constantly switching between the 1st and 2nd shift.