The kinetic diameter of water is smaller than helium because its polar nature allows it to adsorb and diffuse across solid surfaces.
This behavior helps it fit through small holes better than helium can.
This behavior helps it fit through small holes better than helium can.
Which is to say don't think about the filter as a geometrical sieve that has holes that pass molecules solely based on their size (either it fits or not).
This is not true even at macro level.
Has to do with how to define the diameter of a molecule/atom when you consider how it interacts with other matter.
Atoms are fuzzy since they are mostly electrons moving about, thus there are a few different ways to define diameter depending on how this cloud behaves.