22/7 is just a close approximation for pi, not pi itself.
If you put 22/7 into a calculator, you get:
3.142857...
While the number pi itself is:
3.1415926...
You can notice that pi is actually different than 22/7, even though the two numbers start out the same and are close to each other. Frequently in geometry, you use 22/7 as pi because it's so close and makes calculation easier.
