I am not much of a "maths-guy".
Just a thought (that would possibly explain some things OR sum up your post in some lines):
Would the math be right (75/25) if you calculate the curation into the rewards of the author?
Payout author = 75% + author curation reward
Curation (others) = 25% - author curation
The opportunistic conclusion to this would be: Stop voting for selfvoted stuff + look for really good content + Selfvote only just before payout
The best solution for steem would be: Stop selfvoting (even with sock puppets AND boosters) + vote what you really like