These two teams last met June 19, with the Cubs winning 7-3 at Wrigley Field.
Since then, the Cubs are 6-9 and the White Sox 6-6 and therein lies one tale of why the Cubs’ record is worse. The Cubs have had just one off day (Friday) since that series and the Sox have had three. Cubs off days have not been spaced out well this year; quite a number happened in April and early May, not many since then. They’ll be spread out a bit better in the second half.
That’s not to use them as an excuse for the Cubs’ mediocre play lately, but playing 50 games in 52 days is not optimal.