Circumference = Pi * Diameter
Diameter = Circumference / Pi
If we use 3.14 for Pi, we get 12 / 3.14 = about 3.822 for the diameter.
If we us 22/7 (about 3.1428) for Pi, we get 12 / (22/7) = about 3.8181 for the diameter.
If we use 22/7 instead of 3.14 for Pi, we're using a larger value for Pi and getting a smaller result (3.81 vs. 3.82) for the diameter.