Dear Cristina,
thank you a lot for the reply and clear explanation. I am getting familiar with this system, hence I miss many things. The takeway from your explanation, besides clarifying the issue itself, is that the WDQMS provides the status of the observations (availability and/or quality) with respect to one or more GPC within the group (ECMWF, NCEP, DWD and JMA). I believe I am not the only one thinking that these figures, i.e. availability and quality, are just there like magics and do not know the actual process behind.
Cheers,
Alessandro