Yeah, variation is normal. Think about it, us humans get variation in our stools too even when there is nothing wrong with us. It's just one of those things.

Obviously, the it's like water and keeps coming out like that or had blood/mucus whatever, then it needs to be addressed but, otherwise, don't worry.

Sometimes, all it takes is for the dog to be slightly stressed out by something. Adrenalin can cause stools to be loose. My dads greyhounds pass these loose stools after they have just had a run. It just causes food to be rushed through the digestive system faster than usual which is why it comes out as diarrhea. Thinking about it, when I am stressed, the same thing happens to me.
Another possible thing is dogs who pick stuff up when out on walks. Sometimes, they might eat something that upsets them slightly but it's usually nothing major.