If you have old electrolytics that haven't been charged for a decade or more, you need to "reform"
them by gradually raising the voltage and monitoring the leakage current, allowing the electrolytic
reactions to complete again - otherwise severe leakage can happen and overheat or damage the
capacitor. This is more important for high voltage capacitors where the power dissipated in the
leakage can be considerable.
This reforming happens whenever a cap is powered up, but to a much lesser extent if its been used
recently - which is exactly what your measurements show.
The actual insulating layer in an aluminium electrolytic is the anodized surface of the aluminium
foil, a layer of alumina a few nanometers thick for low voltage capacitors. Since alumina is an
extremely good insulator, and since the layer is incredibly thin, you get very high levels of capacitance
in a small cap. The applied voltage maintains the layer from slow degradation by the electrolyte,
hence the reforming process.