This is an age old problem. Some batches seem to worse than others and it affects a multitude of brands.
The problems are 2-fold really ... The speed if sound is not a constant. For instance, different levels of humidity will affect it one way or another as will other factors.
Manufacturers seem to like to keep their 'Sub-sonic' ammo running pretty close to the lower variable of the speed of sound for maximum performance. The tiniest variation in loading will pop the bullet out faster than Mach 1. (Speak to any home-loader to find out how even the tiniest variations in load affect ballistics

)
So ... we have non perfect loads operating at close to Mach 1 for the best part, but high humidity and small variations in loading will lead to some of that ammo producing super-sonic bullets.
It wouldn't happen in a perfect world, but since when did we live in one of those ?