[morzh]

Unloaded means all or some of the tubes are not inserted, so there is no load or less load than the nominal one. This will bring the voltage up.

Another consideration is, if the nominal voltage is the one listed in the documents, and is measured with today's instruments like a DMM, it will show a higher value due to the fact that the old instruments loaded the circuit having low input impedance whereas the new instruments having very high impedance do not load the circuit being measured.