The Brewers won that game in Colorado by 15, they lost the Skubal game by eight.
Take out the biggest outlier on either side and their run differential is still +25 for the other 14 games (10 W - 4 L) this month.
But ultimately the run prevention floats the boat.
In the eleven games the Brewers won this month they gave up zero runs (x4), one run (x2), two runs (x4) and three runs.
In the five games they lost they gave up five runs (x2), seven runs, nine runs, and eleven runs.