Now, in the case of 700x35c tires, Sigma's instructions for BC 1606L computer are to enter the number "2205", while Sheldon Brown's instructions are to enter "2168". Both numbers are supposed to relate directly to the wheel's circumference in millimeters.
Why the discrepancy?
I honestly don't know. I took a look at your link to the 1606 and I couldn't help notice that for 700X35C they also list an ERTO size of 37-622. So that number could be used for either a 35 or 37mm tire. I went one step further and looked at cateye's calibration chart and they have 2170 for the same tire size. When these charts are made by the various manufacturers I am not sure what method they use to come up with these number. I doubt they buy all sorts of tire sizes and do actual measurements. My guess is they obtain some manufacturing specs and do the math. Given the descepency in tires sizes I am not surprised there are different numbers for calibrating a computer.
Let's not get too wrapped around the axle here. This sport is supposed to be fun and getting yourself twisted out of shape over a few millimeters of calibration just is not worth it. Even if you use a number that is off by several MMs you still have more accuracy than is reqired by law in an auto's speedo/odo. In the end the difference between Sheldon and sigma is only 37mm (1.4 inch). Is that really something to worry about?
I stopped sweating over this kind of thing years ago and accept the charted data. I feel a lot better and have more time to play with my son.