Photoionization cross sections calculated up to the 20th single ionization threshold of triplet $P$ states of planar helium exhibit fluctuations. These are mainly due to a dominant series of resonances which can be associated with an approximate quantum number $F=N\ensuremath{-}K$ in accordance with three-dimensional full calculations and experimental observations. As the energy increases the dominant role of a single series as sole contributor is apparently lost as new series start to contribute significantly to the cross sections. This would result in an earlier onset of Ericson fluctuations as in the picture of a single dominant series, where the onset is expected around ${I}_{34}$.