I've noticed in several games I have running now, that many of my planet's populations will simply stop growing before they even reach the planet's population cap.
The first planet I colonised, for example, the 2nd one given to you in your home system, has stopped growing at 2.4 billion (when you zoom into the planet, this is the figure given), but when you simply click on it, the population is registered in the details as 2.39.
Could this be a bug caused by my tendency to land a colony ship with 250 colonists, then double click to have it exit the atmosphere with 1 colonist in the same turn?