A cumulative, sporadic 20 makes more sense. I can understand that. There were years when I did hardly any active collecting, except on the one weekend each year when I went to WESTPEX with my uncle.
By the way, there's no t in erilaz, not even in the (presumed) pronunciation. It's a Proto-Germanic or Runic Norse z, not a German z. The sound represented by the runic letter in question is sometimes even transcribed as R (thus erilaR), since it was on its way to becoming r in Old Norse.