I have got one way to achieve my final goal:
Answered here by Jennifer Petti
Re: Finding Difference From Reference Line and Coloring Above & Below Line
But still looking for the answer of my first problem to understand the avg calculation clearly.
If you have 492 orders for [Product Sub-Category] Appliances, then:
COUNT([Product Sub-Category]) = 492
Are you looking for the distinct count?
COUNTD([Product Sub-Category]) = 1