We have compiled the intensities of the HeII 4686 lines measured in Galactic planetary nebulae. We present a few observational diagrams related to this parameter, and discuss them with the help of theoretical diagrams obtained from simple model planetary nebulae surrounding evolving central stars of various masses. We determine the hydrogen and helium Zanstra temperature for all the objects with accurate enough data. We argue that, for Galactic planetary nebulae as a whole, the main cause for the Zanstra discrepancy is leakage of stellar ionizing photons from the nebulae.