I use Google Data Studio (GDS) to process some data gathered by PiwikPRO, most values are transferred accurately, but some have a considerable mismatch:
I just checked and unfortunately I cannot reproduce your issue - the numbers seem to match what I see in my Piwik PRO account. I used the same conditions and the same date ranges across both products.
Another important thing to consider - did you build the charts the same way as you would build a widget in Piwik PRO dashboards? For the chart showing “returning visitors”, did you set it up by using the metric “Visitors” and making a scorecard filter like on the screenshots below?
Is my guess correct that you are comparing stats for a particular location? In the screenshot from Piwik PRO Analytics, I see the following text: “Country › Subdivision › City”, which suggests that you are looking at the Locations report.
If that’s the case, it’s important to make sure that you filter the data the same way in Google Data Studio. By default, Data Studio would return the average session time for all visitors without taking locations into account. If you add locations to the mix (like in Analytics reports), the data may be calculated differently (to split between cities and also to see results where location couldn’t be attributed).
In general, it can be pretty hard to compare a scorecard chart in GDS to a full report where the query for the data is more complex. A useful tip might be checking if the data is accurate by comparing it to a widget in Piwik PRO dashboards.