What is "plain" ice cream?

Since when have words and sentences always meant what they were supposed to. The way I see it when we see Plain Ice Cream we think of Vanilla Ice Cream. In this case Plain does not refer to no flavour but rather Ordinary. What I am basically saying is that when Plain is added with Ice Cream it no longer means no flavour. On it's own it does. Think of Plain Ice Cream as a name rather than a word added with a name (If you know what I mean). That's how I see it anyways.