I have now been told by them that it's a matter of recreational players vs non-recreational players. I can't really trust any information from the idiots working the chats, but I believe that's actually what has happened here, what they deem recreational players see all selections, what they deem non-recreational see only some.
I fully believe this was not meant to be applied like this for a big market like a tennis grand slam (the rec vs non-rec is usually for obscure stuff like Russian table tennis), but now that it has been set that way it's quite the challenge to get them to understand this and change it. I know this because it's happened in the past that they've set some markets for recreationals only, but once the info has gotten to the right person they have changed these settings, so that the markets have become available for all players.
Thanks again biprob, sportsbettor and ace for sharing this info. I believe I have all the info I need now.