You are outputting the average session duration data for GA in minutes with only two decimals such as this:
Converting this decimal to seconds (*60) gives this number:
If you display that as mm:ss you get
Problem is - that is not the correct number - since you only pull/display the minutes with two decimals you don't get the actual number of seconds which is:
Change that to the mm:ss format and you get
Why would you output the number like this? It leads to discrepancy between looking at the GA interface and your numbers - not confidence inducing.
This is the format how the data source API delivers it. It is unfortunate and our development team is looking if there's something that our system could do to change the format it's brought to the table.
Thanks for letting us know!