That view is showing you all email channel engagement from yesterday. It's not bound only to emails that were sent 'yesterday'.
Looking at the 'previous period' comparison, it looks like you sent a lot more emails the day before yesterday. Out of those 214k sends, some emails were opened the day before yesterday and some were opened yesterday. They're still unique email opens and they still happened yesterday. It's just spillover engagement from the day before.
If you want to view metrics based only on the campaigns sent 'yesterday', you can use the new report builder to build a custom view. You can then add all the relevant metrics, channels and other criteria.
