Karma doesn't accumulate linearly. I don't know exactly how it works, but certainly for articles I've posted it's true. It might get 300 points but your karma only increases by 200. I don't know if this non-linearity has changed over time, but it could certainly be used to achieve this effect.

And to be clear for others who are wondering what happens at 500 points, it's a threshold for being able to downvote, not a limit per se.