Voltmeter is an instrument which measures electric potential difference between two points.
When measuring electrode potential of some redox system (vs SHE for example), it is said that voltmeter reading contains sum of all potential differences present in a cell. This includes all electrode/electrolyte potential differences, contact potential differences between electrodes and probes of voltemter and possibly liquid junction potential. Probes of voltmeter are connected with two electrodes in a cell.
However, since voltmeter measures potential difference between two points, how can it sense sum of all potential differences in a cell if it is connected only between two electrodes and not all potential differences are created at these two points?