99% of the time you can take it as "I dislike like you to a smaller or larger degree". Taken on face value, without regard to how it's used, it actually means "The amount I like you is not especially high". But that's not what's meant. Here's another similar case that might shed some light. You know how people say things like "He's not the most handsome man in the world"? It's a gentle, indirect, somewhat diplomatic way of saying that you don't find him attractive. It could even be used to mean, explicitly, that you find him decidedly unattractive. And when you think about it, there is only one individual on Earth, out of billions, who is "the most handsome", so saying that someone isn't him is, on face value, saying virtually nothing. Hence, the subtext.