Some points:

1. Breakage in Travelers Checks for one very large issuer is running at roughly 2.1%, and they're as good as cash, but some people never cash them.
2. Breakage in Time and Savings Deposits has sometimes been as high as 1/2%, with people either forgetting or never redeeming.
3. In loyalty programs the ratios have been much higher, often higher than 20%, sometimes well above that.

Most loyalty programs have three basic causes fro breakage:
1. Small balances that are simply abandoned;
2. Balances that have been allowed to expire, some of which can be very large (a recent example, obviously an outlier, had 1.7 million miles expire in an airline programs I examined)
3. Unreported death, divorce or other status change that ends out with abandonment.

There are obviously many other specific reasons. Many studies of breakage in bank accounts, tax refunds, ticket refunds/credit vouchers, loyalty programs etc have been made.

BTW, in some multiple participant loyalty programs (eg UK Nectar) the program manager keeps all breakage, thus wildly increasing program profitability. Most airlines make much of the money they make on their mileage programs by selling points as if redemption would be 100% then having actual breakage rates of 20% or more.

They really don't need to deflate the miles to make a lot on money on these programs. It's only people like FTers who tend to watch their balances well enough to keep them.