Mere stupid copying may perhaps be added. An example of it appears on a map printed with a pamphlet entitled Conquest and Kultur, compiled by two college professors and issued by the Creel press bureau during the Great War. On this map, borrowed from an English periodical called New Europe without correction, annex is spelled annexe. In the same way English spellings often appear in paragraphs reprinted from the English newspapers. When used as a verb the English always spell the word annex.
Annexe is only the noun form.
A copy of this style book is in the proof-room of nearly every American daily newspaper and its rules are generally observed.