That's how metaphor works. It actually happens a lot in English, here's a list of so-called "contranyms" http://www.dailywritingtips.com/75-contronyms-words-wit

That's how metaphor works. It actually happens a lot in English, here's a list of so-called "contranyms" http://www.dailywritingtips.com/75-contronyms-words-with-contradictory-meanings/ http://www.dailywritingtips.com/75-contronyms-words-with-con... EDIT I'm actually not crazy about this list, some them are pretty tenuous.