The volume restriction in the safe harbor allows an exception for block purchases. That can increase a program's flexibility significantly. To qualify as a block, the stock purchase must have at least one of the following characteristics:

* A price of $200,000 or more.

* At least 5,000 shares and a price of $50,000 or more.

* At least 20,000 shares and 150% of the stock's average daily trading volume (excluding block trades) for the preceding four calendar weeks.

"So if there is a big chunk of shares out there--and often these are privately negotiated trades that are not on the market--that trade isn't included in the volume restriction," Donegan says. "The idea is that there isn't the opportunity for market manipulationMarket manipulation describes a deliberate attempt to interfere with the free and fair operation of the market and create artificial, false or misleading appearances with respect to the price of, or market for, a stock.
..... Click the link for more information. when the company is buying a large block back from one stockholder" In fact, if such a large block were to be thrown into the open market, it would probably cause a supply--demand imbalance, forcing the stock price down. That would not be good for the other investors.