Custom Fields Available for Customer [Sales Detail] Report

I have created a custom report cloned from the "Sales [Customer Detail]" report.

There are 6 x Custom Fields available, listed as follows (yes, there are 2 x sets of custom fields):-

Custom Field #1

Custom Field #2

Custom Field #3

Custom Field #1 (populated from customer card)

Custom Field #2 (populated from customer card)

Custom Field #3 (populated from customer card)

I have populated the set of custom fields in the customer card record as follows:-

Custom Field #1 - value: "Salesforce ID"

Custom Field #2 - value: "Division"

Custom Field #3 - value: "Branch"

The report extracts the above values from the customer card correctly, however, it's unknown from where the other set of values is extracted.

Q. From where does the report retrieve values for the other set of "Custom Field" fields?  

Refer to attached set of screenshots.