Oh it's possible that the runniness you're referring to is simply the way the natural pb is without the added sugar.
I'm not sure since I'm just used to it.
But I do know that it is definitely much more runny than conventional pb. My kids still use it on sandwiches tho. But again, they have never known differently either.
I've also found that the "chunky" version is a bit less runny than the creamy. I buy both because different kids here prefer each variety.