When you sharpen a Spyder with the Sharpmaker, you are adding (or restoring) microbevels to the very edge of the blade. The microbevels
must be at a greater angle than the primary bevels. Spyderco shoots for primary bevels of 15 degrees per side, or 30 degrees inclusive, from the factory. This means you must use the 40 degree Sharpmaker setting to add microbevels.
You will sometimes get a Spyder from the factory (particulary those made in Golden) with primary bevels less than 30 (around 20) degrees inclusive. The 30 degree setting
can be used to add microbevels to these knives. The 40 degree setting still works fine, though. To save yourself much frustration, just use the 40 degree setting unless you know for sure that the knife has primary bevels less than 30 degrees inclusive. This is something I learned the hard way