Struggling to understand series connections based on experiment
Light intensity for incandescent lamps is related to temperature and area of the filament (and probably a couple more factors). 

Small differences in the length and diameter of the filaments can lead to slightly different resistances (which you saw in your voltage measurement). 

That, in turn, can lead to a somewhat different power consumption (P = I * U), and you end up with a noticably different filament temperature, light intensity and color temperature.

TLDR: The lamps are identical only in theory.