V-Core 3.1 Belt-Tension

Hello. I disassembled my V-Core 3.1 300mm again, realigned it and installed new belts. I've had the printer for 3 years now. I was already confused about the belt tension at the time. Now I notice it again. Before installing the belts, I aligned the X axis at the front of the idlers. After belt measurement tension several times, I have approximately the same tension on both belts. But now one side is pulled back again. This means that the X axis is no longer align to the idlers. It's always been that way. What didn't I understand? Can someone please tell me the reason for this? Thanks.