Years later it turned out some of the giants had classes / types and the title was a reference to the Attack type of giant. Thus the English title would've been better as "The Attack Titan", and indeed the Japanese title could also have been interpreted as that, though it's only obvious in hindsight. The Japanese title was likely deliberately intended to have the double-meaning "Attack of Titans" and "The Attack Titan", though this double-meaning cannot be conveyed in English, and in fact we're now stuck due to inertia with a third English rendering that is completely disconnected from either meaning.