Question 911990
An explanation I can give is 

The circumference is pi * D
where pi is a constant = 3.14

you are actually measuring the circumference   pi * D1 & pi*D2  with measuring tape
where D1 is the diameter of the outer pipe
and D2 the inner pipe.

what ever are the sizes of the pipes the difference is a multiple of pi.

when you are dividing by 6 

you are approximately dividing by 3.14 *2 or 6

when you divide by 6 you get the gap between the pipes.