According to the link below, and other sources which seem to have copied its ideas, "variety" means that "sentences should differ in length and form". Personally I don't see what this specifically has to do with paragraph construction. It seems to be more a general point about writing style.
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CalifJimUnity and variety are opposites of a kind. That's the point. There's a conflict between being too repetitive (too much unity) and stating too many different points (too much variety) all in one paragraph. Therefore, as in any art, you have to strike a balance. Don't keep harping on the exact same thing (not enough variety), and don't go off in all directions sayin
What a variety mean?
anonymous i am learning to write paragraph writing. I studied that a paragraph should have unity, order and variety. I can't understand what this variety means bcz unity is forcing us to stick to the main topical sentence but variety is almost an opposite