co-vary vs vary

co-vary and vary both are verbs.

Word	Noun	Adjective	Verb	Adverb
co-vary	No	No	Yes	No
vary	No	No	Yes	No

As verbs, vary is a hypernym of co-vary; that is, vary is a word with a broader meaning than co-vary:

co-vary (verb)	vary (verb)
vary in the same time period (of two random variables)	become different in some particular way, without permanently losing one's or its former characteristics or essence
	make something more diverse and varied
	be at variance with; be out of line with
	be subject to change in accordance with a variable