"Evidences" isn't really the plural of "evidence." One piece of evidence and two pieces of evidence are both just "evidence." (The same is true of "literature.") The reason why "evidences" doesn't show up as wrong on spellchecker is because it's registered as a verb. It's a roundabout way to say "shows."
(Please retain the reference in reprint: http://www.letpub.com/index.php?page=author_education_evidence)