and apparently there are enough participants on the forum whose names do not appear in any spreadsheet, nevertheless, these guys wear signatures with advertisements from different bookmakers (I suspect this is not a gesture of goodwill).
It is quite possible that some people have individual agreements with certain companies or casinos like you said. Maybe they are friends with the developers, maybe the sites throw some work their way and this is a way to return the favor and say thanks. Or maybe they just gambled there, got lucky, and they now want to advertise them a little. If I remember correctly, Lauda used to wear a FortuneJack signature for a long tome even though she/he was not a part of the campaign at one point. At least officially. There are surely other examples, I just can't think of any.