I think if you see the difference as a group, and it's not just the result of a random distribution, that suggests there are factors out of your control that are influencing the split. Just eyeballing a minor league schedule, it looks like day games are almost never the first game of a series. So day games would have reduced travel effects, a huge deal in the minors with bus travel. On a similar note, by the time those games roll around, there would be more familiarity with the ballpark/opposition/umpires, etc. (Day games are also always the first game of doubleheaders, which are more frequent in the minor leagues.) Umpiring might be different in day games vs night games too, especially with lower-quality umps under lower-quality lighting? Also, day games are usually preceded by night games, so perhaps backup/part-time catchers play more often those days. And backup/part-time catchers are probably worse at catching at lower levels, which can affect the pitching (umpiring too). If they are often "getaway days" maybe that has an additional effect on manager decisions (less aggressive bullpen use, etc.). Could still be random noise. Just looking at the Chattanooga May 2015 schedule, their two Sunday games (which are most often day games) were won by scores of 14-13 and 16-3. Results like that could have an outsize effect in small samples! (And indeed day games represent ~20% of Walker's career AB, less than half of even home/road splits.)