Most all wall warts used for Nixx type batteries will read a much higher voltage than what is to be charged. They depend on the internal resistance of the charger to drop the voltage down to a proper level to charge the battery that is connected to it. Most of these will read open an open circuit voltage of at least 3 or more volts above the voltage of the battery that you will charge with it. If it reads less, it will not charge the battery at all, in fact may even discharge it depending in what caused the fault in the charger. An extreme example of this is some of the early vintage (1950"s versions) would read 115 volts open circuit as the charger was just an isolation transformer with a rectifier diode and a current limiting resistor in series with the output. The voltage drop across the resistor would then drop the voltage to the required level.
To see what is really happening you need to measure the voltage with either a peak reading meter or use an oscilloscope. The conventional VOM will not give you an accurate measurement as this is usually not a nice clean constant voltage, it is usually either a full or half wave rectified sign wave. It is just a natural physical fact, the voltage of the charger must be higher than the voltage of the battery being charged or you will not get any charging current. You can approximate a peak reading voltmeter by putting a capacitor on the output while you are measuring the voltage with a typical VOM.