Questions

The "average monthly sales All branches" is average sales per product option across all branches over a period of 6 months. This is a pre-calculated field and can be used as a reference to purchase the product based on sales over the period of last six months.

Whereas "Avg. Sales Qty Based on Input Dates - Current Branch" is average sales per product per branch over a period of the user input date range.

Example: 

If the user input date range is 1-Jan-2019 till 10-April-2019:

"Avg. Sales Qty Based on Input Dates - Current Branch" = Number of Units sold per branch/2

Whereas

"average monthly sales All branches"= (Total number of units sold across all branches over a period of last 181 days)/6

Unfortunately, this report only shows the average monthly sales of the main product and not for each of the size products separately.