Sforno on Numbers 35:25/ספורנו על במדבר ל״ה:כ״ה

1עד מות הכהן הגדול, the Torah had already made clear that exile is a penalty applicable only to inadvertently committed killing. Seeing that there are different degrees of inadvertent killing, i.e. some border on accidents totally beyond the control of the perpetrator, whereas others are acts of criminal negligence, though unintended, the Torah in determining the penalty does not give us a fixed time, but a variable. When the killing occurred only a short time before the death of the High Priest, the exile spends little time in the city of refuge; on the other hand, there may be times when the exile dies in the city of refuge before the High Priest officiating at the time of the “crime,” dies. These variables are taken into account by G’d Who alone is in possession of all the facts, i.e. the degree of negligence which caused this inadvertent killing. The important reference to G’d’s involvement and calculations in all this is found in Exodus 21,13 והאלוקים אנה לידו, “G’d had caused, directed his hand,” (that of the inadvertent killer.)