A quick analysis over the last 2 weeks of August shows that the top rewarded users (by percentage rewarded) receive Musing upvotes on up to 70%-80% of their posts. [In fact there were a few users with 100% upvotes but no-one with more than 5 posts in total].

However it is noticeable that these users are averaging around 1 to 4 posts per day on Musing. So my assumption is that these users are producing small numbers of high quality responses. A case of quality over quantity.

For users that post more than 10 times a day the upvote ratio falls significantly, with the top users at this volume receiving a maximum of 20%-30%. 

I think it will also depend heavily on whether the user mainly asks questions or provides answers, and on what kind of questions they respond to. Some questions are fun and only require short answers, but are probably less likely to receive upvotes.