How to calculate Average daily on-hand units?

With the new low inventory fees coming, I wanted to do some calculations to to predict different scenarios.

One thing that has be scratching my head is how Amazon calculates “Average daily on-hand units”.

I have a product that always has 10,000 units in stock, if it ever drops below that amount, I would top it up back to 10,000 units.

However, Amazon says my Average daily on-hand units is 30,000 units?

This does not seem like an “average” to me, and I’ve never had anywhere close to 30,000 units at any given time.

Could anyone shed some light on how this is calculated?