"To reduce by 10%" is, in English, the novel usage. It was apparently invented by essayist Richard Grant White in 1870. The original meaning of the word, "to destroy", is attested to 1663:
The meaning may be obscure and obsolete, but the word does mean something specific.
Rather, I'm sure Caesar used the appropriate conjugation of the Latin verb "decimō".
Two separate, but related, words. With two separate, but related, meanings.