This is one of the biggest concerns for all restaurants, not just BBQ. And there are a variety of things that can affect consistency. From the various cooks that are working to the source of the food being used. There are always variables in play that can change the end product.
With BBQ, one of the added issues is pit management. Even the best cooks can have bad days when it comes to the fire, temp control, etc. Obviously, experience helps even that out over time, but bad cooks can and do happen.
And then there's just poor management. You get food reps hounding you all the time about why you should switch to their product and how they can get you something for 20 cents cheaper, and some people will just see dollar signs and switch to another product without fully realizing how it's changed the end result. Or they hire the wrong people and then on the owner's day off these idiots don't give a damn about maintaining standards and you get food served that shouldn't be, etc.
It's just a tough industry and even the smallest things can affect the end product, and when people don't get the same quality twice, it's not often they come back.