I'm thinking the bank teller needed $100 and felt he had an easy mark who wouldn't notice. Easy to put it down to adjusting an error, but seems suspiciously like he pocketed the $100 himself. If OP is as calm as she says about not raising a fuss, who easier to steal $100 from? If this bank has so much inept performance from their staff, and they excuse a teller who does this sort of thing, it's definitely time to find a new bank.
I knew a guy once (about 1980) who worked in a convenience store. He earned minimum wage for the work. But this guy always had a ton of pocket money, and seemed to have a lot more disposable cash than his job would indicate.
I finally got him to tell me what he was up to. As I remember it, basically when someone gave him exact cash for a sale, he'd take the cash and give them the item. At sometime after the customer left he'd ring it through the register for five or ten dollars less than the price. He'd track his deliberate shortages, then at some point he'd pocket the amount he'd accumulated. By the end of his shift he had a pocket full of cash. Sneaky bugger.
This went on for a few months, till the management began to notice that his shift never seemed to bring in the same kind of money as the other shifts at the store. One thing led to another, and the guy quit and left town shortly before the sheriff came around with a warrant for his arrest. I never did hear whether the guy was arressted or not.
Criminals are all around, and some bank tellers are no exception.
Dave