Because he should have remained in the city of his refuge until the death of the high priest: but after the death of the high priest the slayer shall return into the land of his possession.
Read Chapter 35
Pacian of Barcelona
AD 391
What does it mean that a homicide returns for absolution after the death of the high priest, except that the human race, which brought death upon itself by sinning, receives absolution for its guilt after the death of the true priest, namely, our Redeemer? Exposition of the Old and New Testament, Numbers