Just created a new LookerStudio dashboard, and tried my luck with reporting daily sessions.
The metrics are wildly different.
What am I doing wrong here? 
Did you do any connector updates recently? Based on the aggregation type for the “Sessions” metric (shown on your screenshot as “AUT”) I’m guessing that you’re using an older version of our integration. Google changed the way for aggregating data in one of the latest Looker Studio versions and we had to introduce some changes.
I’m seeing correct numbers on the latest version of our connector:
Here’s how to update the connector currently: Update Looker Studio integration. Once we are published in Google’s Connector Gallery, the update process will be simpler.