If you check your receiver settings after some sort of "auto setup"(Audyssey for example) and it sets the subwoofer further than seems correct. Don't automatically assume it has made an error.
Most high performance subwoofers use state-of-the-art micro processor controlled amplification these days. This tends to increase efficiency, longevity, and overall sound quality for reasons I will dive into at a later date. For now I'm going to focus on DSP enabled amplification and how this relates to the distance setting in your receiver. The correlation is fairly simple. The DSP(digital signal processing) will impart a signal delay usually measured in milliseconds. A common delay would be 5 milliseconds. The speed of sound is (roughly) one foot per millisecond. So a 5 millisecond delay will cause your receiver to set the distance setting about 5 feet more than the physical distances involved.
It may seem counter intuitive at a causal glance but the larger the distance setting, the LESS of a delay the subwoofer signal gets. Example. You have two subwoofers in the room and one is 3 feet from the seating position and the second sub is 10 feet from the seating position. The second subwoofer(10 ft away) will have its output arrive approximately 7 millisecond later(remember, one foot = one millisecond). We cannot "speed up" the speed of sound(this isn't star trek) so we have to delay the subwoofer closer to the seating position as needed.
The distance setting is important as a correct setting will ensure a seamless "blend" between the subwoofer and your main channels at the seating position(s).
Power Sound Audio