I imagine the types of things you do with the respective languages as well as the number of people in the communities have a pretty big impact.

For example if you're using c++ to implement an interface for some hardware then maybe you can just quickly follow a spec. But with lisp you're more likely to try and implement some sort of ai thing that requires a lot of stopping and thinking.

With respect to community size, if you have a bunch of people then you're going to get more accurate numbers but if you have a small community then you might end up with a lot of outliers.

Finally people are probably more likely to have a significant amount of practice with c++ vs lisp and that probably has an impact.