Tracking Sales Returns over Time

Hi, 

I am trying to replicate certain analysis I perform on Excel relating to the aging of Sales Return. I have come pretty close, but with gaps. 

I have Sales and Sales Return data for Capital goods in one table as follows: 

 Note: Year of Sales and Sales Return Months have been created from Sales date and SR date in my source (not in above display). 

Using the above data, I create the following output in Excel which shows the Cumulative Sales Return amount against the 'Year of Sales' (Year when sales originated) in Rows and 'Number of months' in Columns: 

Note: For sales that originated in 2014, the first Return was 6 months later (1/06/2014). Hence, we have the SR amount of 3000 populated against 2014 and 6. The next such return occoured 13 months later (worth 4k in 1/01/2015) and the next 14 months later (worth 10k). These were accordingly cumulated. Similar analysis for 2015. 

To replicate the same in Power BI, in my Sales table, I created a calculated column that returns the difference between 'Year of Sales' and 'Sales Return Month' in months and added 1 to it. Let's call this column 'Month_Diff'.

Furthermore, dragged this field to the Column field in a matrix and the 'Year Sales' to the rows. Added a quick measure for cumulative sales return: 

However, the output I am getting is close, but not exact: 

Any help would be appreciated!