Power Supply Unit (PSU) Uncertainty indicates an uncertainty in the power supply path as to which phase is carrying the load from a server or chassis.
Many servers distribute the load evenly across multiple power supplies; some power supplies are most efficient when operated at a higher load. This gives an incentive to turn off power supplies for servers or blade chassis with multiple power supplies, meaning the real consumption may be higher on some power supplies than if the load was evenly distributed.
Since ITA doesn't model power supplies, it is unknown which power supplies may be turned off by the server or blade chassis. If the server or blade chassis is connected to different phases, there is uncertainty as to which phases are carrying the load.
To make sure you do not exceed your capacity, ITA reserves some of the capacity for PSU uncertainty which you can see expressed as PSU uncertainty in the power tool tips.