You were right, I happened to be double-checking pictures and realized they are using the SX700-LPT platform.
Tony mentioned that the fan on the SX800-LTI starts at a lower load level than the SX700-LPT because the 700W is actually more resilient to higher temps at lower loads than the 800W. So be quiet! is likely sacrificing wattage and guaranteed efficiency to optimize strictly for noise.