Ok I got confirmation from Engineering. The Errors Per Interval is a metric which is stored in your Smartstor DB and is bound to the Smartstor tier settings in the EM properties file.
The error snapshot data is from Error Detector and that is stored in the traces db and is bound to the ntroscope.enterprisemanager.transactionevents.storage.max.data.age setting.
So the metric above should have historical data prior to 7 days ago depending on what your tier settings are. If you are using Error Detector, then the snapshots will only be kept for 7 days according to your setting above.
Let me know if this helps.