If ever a reliable method for measuring actual intelligence rather than IQ is invented I imagine we’ll be seeing a somewhat lumpier graph than that smooth mean distribution curve.

No, this is how a graph showing quartiles will always look because quartiles, by definition, always include a fixed percentage of the studied population under them.

In this case the lower quartile will always have 25% of the population under it, 50% under the second quartile, and 75% under the third quartile.

Quartiles break a population into 4 equal portions.