alt131 - 4:42 pm on May 25, 2011 (gmt 0)
ooops, cross post.
The answer is in the para before that list in 10.3.7:
If all three of 'left', 'width', and 'right' are 'auto': First set any 'auto' values for 'margin-left' and 'margin-right' to 0. Then, if the 'direction' property of the element establishing the static-position containing block is 'ltr' set 'left' to the static position and apply rule number three below; otherwise, set 'right' to the static position and apply rule number one below.
... and the calculation of width is relevant because the offest for auto (as explained in 9.3.2) depends on what other properties (in this case wdith) are auto as well.