Question about monthly/yearly averages

[Country4ever]

I know I'm at a disadvantage because I don't have software. I do check my stats off the machine every morning.
I don't quite understand this: If every morning my AHI number is 7-8, how come the monthly/6 months/yearly average comes up as 5.2??
How can it do that, if I've never (or rarely) had an AHI that low?
Thanks.

Resmed S8 autoset Vantage
swift original nasal pillow
4Hi humidifier