Actual centrism is nothing more than the state of not falling completely in line with the values/positions of one 'wing'. Anyone who could be accurately described as "right-leaning" or "left-leaning" is by definition a type of centrist; that's why it's 'only' a "lean".

The way centrism is stereotyped is similar to how bisexuals most commonly experience biphobic stereotyping. Bisexuals can be seen as 'gay in denial' (see the 'conservative in disguise' comment you replied to), as sexual 'fence-sitters', as impressionable 'sluts' who don't have the conviction to commit to one sex, etc.

The fact is, a lot more people than one might assume, 'deviate' from both extremes to some degree.