My guess is that pitchers like their routines. And it makes no difference whether or not that routine makes sense or not. I don't know if it means anything, since I don't know if the numbers are polluted by openers, injuries, etc., but these are the 4-, 5-, and 6+ days of rest:
4 Days,GS
3.98
5 Days,GS
4.40
6+ Days,GS
4.19
Maybe it has something to do with scheduling a BP exactly in the middle of starts.