Both Uses

vary
in
Miss Peregrine's Home for Peculiar Children
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The letter was handwritten on fine, unlined paper in looping script so ornate it was almost calligraphy, the black ink varying in tone like that of an old fountain pen.†
p. 61.9 *

varying = differing; or changing
It was as if the constance of their lives here, the unvarying days—this perpetual deathless summer—had arrested their emotions as well as their bodies, sealing them in their youth like Peter Pan and his Lost Boys.†
p. 170.2

unvarying = consistent or the same

standard prefix: The prefix "un-" in unvarying means not and reverses the meaning of varying. This is the same pattern you see in words like unhappy, unknown, and unlucky.

Definitions:

(1) (vary)
to be different, or to change

Vary is often used to describe small differences or changes--especially about things of the same type. It would be more common to say "The weight of full-grown elephants varies depending upon diet and other factors," than to say "The weight of elephants varies from that of mice."
(2) (meaning too rare to warrant focus)