Engagement Rate is calculated as the ratio between impressions and user activity.

When this is measured across multiple campaigns, adsets, or adverts, it’s important to remember that we sum every impression and activity before we divide.

For instance, when we have an individual campaign (as per the image below:

But once we factor in a second campaign with additional impressions, we see a different rate:

The additional 79 impressions in the second campaign lowered the Total Engagement Rate.