Technically, block times should be a Poisson distribution. The reason why they are not is network latency. Hence, instead what I think we’re seeing is the distribution of x+y, where x (latency) is a normal distribution and y (time to mine after seeing the parent block) is a Poisson distribution.
This seems to suggest network latency is somewhere around 3 seconds, which seems to confirm where the ~15-20% uncle rate is coming from.