Average function giving different result per row

Hi,

I'm trying to get an average from a column but is giving a different value every row.

I'm unsure how to find the average for the entire column.

The data source is a salesforce report I created with some mock cases.

I've created a new column to calculate the average: 

Average IRT= AVERAGE('Closed cases'[Initial response business hours])

 

Output:

Anyone have an idea how to get one value for the entire column?

Thanks!