@martin77 Each posting station is counted only once per month, no matter how many stations it hears, so I think that it most likely is due to a lack of stations monitoring the WARC bands.
But to be sure, one would have to do a fairly thorough statistical analysis, and I'm not actually sufficiently motivated to write the code :-) There's always so much to do! (And I like to leave some problems for other people to work on :-) )