Floating Source Voltage Difference to Earth
Floating means that there is a high impedance with respect to ground. However 'high', in practice, does not mean infinite. 

If you attach a light bulb between the neutral and ground it will be unlikely to light and will most likely reduce the measured voltage to close to zero. However with a typical voltmeter that may have a 10M or higher input impedance, the measured voltage may be pretty much anything, depending in part on where that 120VAC comes from. 

That's because there will be capacitive and possibly resistive coupling to both sides of the 120VAC and possibly to other voltages, depending on the circuit. There could also, in theory, be a static buildup of relatively high voltage. Usually the measured AC voltage on a real meter will be less than half the mains supply voltage. 

Think of something like this: 







simulate this circuit &ndash; Schematic created using CircuitLab

If C1 == C2 and the meters are ideal (and the transformer is ideal) each meter will read half the transformer secondary voltage.