What do leader titles mean?

I saw a weird title on Maria being "Mistress",she took tradition honor and aesthetics,is this the title for aesthetics?

Yup, Master/Mistress for Aesthetics.

And Captain for Exploration. That's for the two new policy trees added in BNW.
 
I believe it choses the one with most policies or the recent one, which is kinda annoying. I want to chose it myself dammit!

Also female leaders are still called "lord" with tradition. Minor, but extremely annoying.

That's actually often correct, at least in Britain. For example, Elizabeth II is the Lord of Mann among her many other titles.
 
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