SAN FRANCISCO — Manuel Villanueva realizes he has been getting a pretty good deal since he signed up for Netflix Inc.'s online DVD rental service 2 1/2 years ago, but he still feels shortchanged.

That's because the $17.99 monthly fee that he pays to rent up to three DVDs at a time would amount to an even bigger bargain if the company didn't penalize him for returning his movies so quickly.

Netflix typically sends about 13 movies per month to Villanueva's home in Warren, Mich. — down from the 18 to 22 DVDs he once received before the company's automated system identified him as a heavy renter and began delaying his shipments to protect its profits.

The same Netflix formula also shoves Villanueva to the back of the line for the most-wanted DVDs, so the service can send those popular flicks to new subscribers and infrequent renters.

The little-known practice, called "throttling" by critics, means Netflix customers who pay the same price for the same service are often treated differently, depending on rental patterns.

"I wouldn't have a problem with it if they didn't advertise 'unlimited rentals,' " Villanueva said. "The fact is that they go out of their way to make sure you don't go over whatever secret limit they have set up for your account."

Netflix didn't publicly acknowledge it differentiates among customers until revising its "terms of use" in January 2005 — four months after a San Francisco subscriber filed a class-action lawsuit alleging that the company had deceptively promised one-day delivery of most DVDs.

"In determining priority for shipping and inventory allocation, we give priority to those members who receive the fewest DVDs through our service," Netflix's revised policy now reads. The statement specifically warns that heavy renters are more likely to encounter shipping delays and less likely to immediately be sent their top choices.

Few customers have complained about this "fairness algorithm," according to Netflix CEO Reed Hastings. "We have unbelievably high customer satisfaction ratings," he said recently. "Most of our customers feel like Netflix is an incredible value."

The service's rapid growth supports his thesis. Netflix added 1.6 million customers last year, giving it 4.2 million subscribers through December. During the final three months of 2005, just 4 percent of its customers canceled the service, the lowest rate in the company's six-year history.

After collecting consumer opinions about the Web's 40 largest retailers last year, Ann Arbor, Mich., research firm ForeSeeResults rated Netflix as "the cream of the crop in customer satisfaction."

Once considered a passing fancy, Netflix has changed the way many households rent movies and spawned several copycats, including a mail service from Blockbuster Inc.

Netflix's most popular plan lets subscribers check out up to three DVDs at a time for $17.99 per month. Customers return DVDs in a postage-paid envelope. Netflix then sends the next available DVD on the customer's wish list.

Because everyone pays a flat fee, Netflix makes more money from customers who only watch four or five DVDs per month. Customers who quickly return their movies to get more erode the company's profit margin.

Although Netflix consistently promoted its service as the DVD equivalent of an all-you-can eat smorgasbord, some heavy renters began to suspect they were being treated differently a few years ago.

To prove the point, one customer even set up a Web site — www.dvd-rent-test.dreamhost.com — to show that the service listed different wait times for DVDs requested by subscribers living in the same household.

Hastings said the company has no specified limit, but " 'unlimited' doesn't mean you should expect to get 10,000 a month."

In its terms of use, Netflix says most subscribers check out two to 11 DVDs per month.