It makes no sense to the casual user, but you can understand why it does it when you learn that Google Translate uses statistical methods to learn. Google feed Translate with articles and pages that have been already been translated by a human, and the program learns the translations of words and sentences from that. The problem arises when the two documents it's taught with differer slightly. With translations, this often happens with currencies, country names (lots of examples of translate screwing those up too) and numbers.