Need help with SAMEPERIODLASTYEAR

Hi! The small table "QTR on QTR Return/Exchange Rate Variance" I am showing the return rate for Quality only, for the current quarter and corresponding quarter in the previous year. In the table above, you can see that the value for Q3 2019 for Quality is 0.56% and value for Q3 2018 for Quality is 0.55%, but in the "QTR on QTR Return/Exchange Rate Variance" the value is showing 0.71% instead of 0.55%. 

To compute the return rate "Same QTR PY (previous year) Return Rate", I am using the formula shown in the original post.  

When I break down the values by month, the return value seems to be correct, however, when I try to show in on the quarterly level in the "QTR on QTR Return/Exchange Rate Variance" table, the value is not aggregating correctly.

As you can see in the above table, the values per month for the month and the same month in the previous year are correct. 

Thanks!