It doesn't help that it seems the vast majority of people think that "centrism" is a political position unto itself, defined by being on the fence about everything and never committing to any stance on any issue.
As opposed to the actual definition, which is just someone whose collective of political positions is such that it's not really accurate to refer to them as either kind of -winger.
It's kind of like the bisexuality of politics. Actually, as I write that, a lot of biphobia, which comes from both the homo and hetero, is kind of rooted in the same bullshit 'logic'. Just like the majority mentioned above see centrists the way I described, biphobes see bisexuals as indiscriminate sluts who are willing to bed anyone and everyone, as opposed to simply someone who is capable of being attracted to members of either sex.
If someone is "left-leaning" or "right-leaning", that is an actual centrist, by definition. Otherwise, it wouldn't be merely a 'lean'.