Breeding animals (Breeding twins?)

When i breed two cows i sometimes get two babies. This is easy for me to track as i keep the cows in pens with two cows per pen. when i return to collect the experience i sometimes find two babies in the pen. I've checked and there isn't any missing from other pens, so it's not likely them glitching through walls.

How to re-create: Breed two cows/animals.