Routing Power
If you access to the technology file, you can find the sheet resistance on each layer of the metal. Those numbers are different on each layer.
Picking the top layer of metal for the power truck, designers are considering the low resistance / low IR drop (the top layers are thicker than the lower layer in most of the processes)
Picking the middle layer, designers are considering the routing resources. For example, using power mesh from the top layer, you can imagine the routing will not be that easy.