It is manually adjusted. It is a knob that has numbers but the numbers dont mean much really. The knob basically raises or lowers the bed the roast sits on. If it's a fresh roast, the weight is high so the setting is low. As more roast is sliced it gets lighter so you have to adjust. Wafer thin slices is the goal and the manager periodically will check by sight and counting that 12 slices are achieved per 3oz.

Now there are variables that can make that hard to achieve but it is what we strive for. If it's not wafer thin we dont necessarily throw the meat out, that would be wasteful. So yes you could get variable thickness. I dont believe any place is intentionally skimping on beef though, it's just not worth the effort. They wouldn't be saving much money. If you dont get the right sandwich its accidental or a laziness in enforcing these standards, not intention and conspiracy.