Sigma uses a formula that takes wheel circumference in mm and divides it by 1.6093. So even if you only look at Sheldon Brown's chart, the difference between sigma's numbers and any other computer will differ. Take a look at Sheldon's chart for 700X35. If you use circumference in mm only (group F), you get 2168. If you were to divide that by 1.6093 you would get 1347. Now look again at Sheldon's chart for sigma (group E) and you see that for 700X35 you get 1347. Sheldon's chart does not really differ too much, if at all. It's just that Sigma uses an "adjustment" to get an accurate readout for computers that will be using MPH. This only applies to their older computers. I noticed that the newer models do not use this adjustment.

BTW .. That 1.6093 is the number you would use to convert KPH to MPH.