Why Measurements of Soldered Components on a PCB Are Inaccurate

Measuring an electronic component while it is still soldered onto the circuit board (in-circuit measurement) is only for reference, because other components connected in the circuit will affect the reading.

For example, if several capacitors are connected in parallel on the PCB, measuring just one of them will not give its true capacitance value.
Therefore, for accurate testing, the component should be removed from the board and measured out of circuit.