Screws are categorized in 3 main ways - the type an size of the driver, the diameter (of the shank) and the length. The numbering system is similar and that may be causing the confusion.
First, is the size and style of the driver used for the screw. In case of Kreg screws, they are Square Head size #2. Square head drivers are available commonly in #1, #2 or #3 with #1 being the smallest. Phillips heads are specified the same way, #1, #2, #3. You can look at a kit of screwdrivers and see that the drivers are marked with a #1, #2, or #3 to indicate the size of the driver.
Next is the diameter of the screw itself - the diameter of the shank.. Typical sizes for drywall, pocket hole screws, machine screws, etc are #6, #7, #8, etc with larger numbers indicating larger diameters. Here's a link to a chart of wood screw diameters : www.mypianoshop.com/Screw_Chart.pdf
Last is the length of the screw as measured in inches, ie 1 1/4" screw.
For Kreg screws, most sizes are also available in either coarse or fine thread.
So for example, the most popular screw sold by Kreg for 3/4" wood is the 1 1/4" inch length, #2 square head drive, and #7 shank diameter. It is available in either coarse or fine thread.
You can go to the hardware store and look at drywall screws, and see how they are marked the same way, except that they typically use #2 phillips head drivers.
hope this helps