Acrylic paint is becoming oily when drying
I don't think anything is wrong with your paints. I have experienced it as well at times, even though I rarely work with acrylics.
Acrylic paints go through different stages when drying, slowly forcing all of their 'volatiles' (water and co-solvents) out. But even after drying it will hold a relatively large amount of water, that can still increase and decrease:



  a level of incomplete coalescence causes acrylic films to be somewhat
  porous, leaving channels that run along the walls of the hexagonally
  deformed particles. These pores are then passageways for water to move
  in and out of the film.


This occurs more prominently in environments with higher humidity, since the evaporation process will be inhibited. This inhibition might cause water escaping from the paint to remain on the outer film of the paint.
Another possible cause is condensation: the water vapour of the humid environment turns to a liquid state as it meets the water escaping the paint film.

As for the differences in the forming of a wet layer on the skins of the various pigments, I think they might be attributed to them reacting differently to temperatures, or altering drying times. More likely, however, is that the batches have varying proportions of water, pigments and additives.

The quote was taken from this page on justpaint.org. It offers a good overview (of the different stages) of drying acrylic paint.