That's about it in a nutshell, Dick. But to answer your question- yes, it is cheaper to run constant overtime than have adequate staff. Think of it as a lot of little business units (sectors) that require, say, 2.3 people. Clearly, if money is no object, you have 3 people and a slight inefficiency. If safety is no object, you have 2 full-timers and make them work overtime to fill the .3. Therein is the constant clash between "business imperitives" (money), and safety.
Thats a huge simplification, I know, but the guts of the problem. To make the analogy closer and from a business viewpoint: Imagine you have a restaurant with 60 tables. You have a core staff of chefs etc, but a variable waiter requirement. Some days you need lots, some days not so many. You never know in advance how many, except for the obvious peaks (say lunch rush then the dinner rush). On top of this, every table has a different menu, and waiters take a long time to be trained in these menus, and can realistically only cope with 3 or 4 tables.
Enter the CEO charged with making more money. he cant get more customers, so has to be cost driven.
He can try and slash any obvious costs, but year after year this gets harder. He can then try and make the waiters more flexible, using part-timers, facilitative arrangements, split shifts etc. Use less waiters, but cope with peaks using overtime. Or try and change the way the restaurant does business by streamlining the different menus (that trick is yet to be perfected- and is the SDE theory).