Oops! I always thought the 0.71 was just to prevent clipping and that same loudness reason was the same thing. Makes sense though, the numbers add up to well above 1.
A sound card has the same digital limitations transfer formats do, with a 24-bit DAC there is a solid max for clipping. How it is done depends on the drivers/audio chip but there isn't anything different about downmixing 5.1 for two 24-bit DACs compared to 24-bit stereo PCM.