My average earn became lower since I joined the clubs. How can that be?

You get paid less in SBD if you are club 75 as long as SBD prints

I noticed 3 lines even 6 words without hashtags is enough to receive high upvotes even with only 800 SP.
Where does this Steem come from?

Mana, that is, the power of the voice. mana can then be, for example, 90-100% which gives more than lower mana
If I understood your question correctly

And another thing that counts is that posts that give below approx. 20 steem in payment both author and curator earn slightly less than one for posts of more than 20 steem in payment