Salt Bridge & Voltaic Cells Experiment
Both your cell and your voltmeter have a resistance. The resistance of your cell depends on the number of layers (among other things). The reading of your voltmeter depends on both resistances.
I. e. if R1 is the resistance of your cell and R2 that of your voltmeter, and U is the true voltage of your cell then your voltmeter will only show U'=U R2/ (R1+R2).