Why Does My Series Circuit Show a Voltage Discrepancy?

[Ry122]

I setup a series circuit with 3 bulbs and I measured the voltages across each one
The potential differences were
A: 1.75V
B:1.6V
C: 1.75V
The sum of these voltages was 5.1V
The voltage measured across all three bulbs was 6V.
What are some possible causes for this discrepancy of .9V?

Also can someone tell me why bulb B's voltage is different from bulb A and bulb C?

 

[TVP45]

Did you use a calibrated meter?
What type of connections are used in this circuit?
Did you measure from A-B, B-C, C-D, and A-D where A,B,C,D represent specific points in the circuit?
Why shouldn't there be a difference of Voltage?