Hello, explanation is easy. Old electrolytic will have a bigger current leakage, i.e. the unwanted amount of current which flows through it. With resistances having high value, the small current is completely eated by the leakage, thus never charging the capacitors. As you are connecting the probes, each having around 1M ohm impedance, you are lowering the effective resistance, thus charging a little.
Solution: replace both capacitors with same value. Andrea
