You're training more than one single unit in each batch. With only the barracks, the training size for each slot is 6, but any armories you have will greatly increase that number. If you want to see what your training size is, click on an armory and it will show the breakdown of what your barracks and armories are contributing to your total training size. The larger your training size, the longer each training slot in the barracks will take. This is not a bad thing; it just means you're producing more troops at once. Upgrading your barracks will shorten the training time for each unit.