Calculating frequency of client visits.

Hey guys, Hopefully someone can help me with this probably very simple math problem.

I am trying to find a calculation for calculating the frequency of client visits to our business. I am looking at two things. The average number of weeks clients leave in between their appointments until they come back and see us. And how many times they visit us a year.

For example:

ClientA visits every 6 weeks, and so comes in 8.6 times a year.

ClientB visits every 4 weeks, and so comes in 13 times a year.

ClientC visits every 10 weeks, and so comes in 5.2 times a year.

The average weeks left between appointments for all clients (6 weeks, 4 weeks, and 10 weeks) is **6.66** weeks.

The average visits a year for all clients (8.6, 13, and 5.2) is **8.9** times a year.

**The Problem**

When I work it back and take the two averages it doesn't work. A client visiting every 6.66 weeks would visit 7.8 times a year. yet the average I got was 8.9.

Can somebody please explain **why** this is?

Many thanks,

b