How do you explain that I have never been upvotes by OCD?

Maybe you have used tags that aren't seen by curators? Maybe there have been a lot of competitions at that time?

Curation projects like OCD and Curie have their own specific guidelines to follow. Maybe your content didn't pass the guideline. Not saying content is not good, but one of the common guidelines of these curation projects is being under-rewarded. Another thing that I know is both projects don't vote for crypto-related content and prefer original-content. We curators work very hard to spread our votes that are of course according to our guidelines. We use different methods to discover content like Steemlookup, manually looking at tags or "New" section, looking through self-promotion in Discords, looking at feeds for resteems, etc.

but I see often the same authors being voted by the projects and the persons behind it.

This is because their content still passes the guidelines.

I hope this clears it up. Cheers! :)

-hiddenblade from Curie