If a lamp is in circuit then it's resistance will change with the heating effect even if small and would cause a difference in reading each time you try.
I just plucked a figure out of the air for the resistance of a lamp
Peter
I will reiterate - your original question was why are you getting a varying voltage - when in fact you were measuring at two different points in a circuit with different reference points which to quote an US mate of mine 'was polluting your results'
As someone has already said stick your meter probes into a socket and take a measurement.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local