The "Expected Value" is essentially a value that is derived from your historical trends.I don't know all the math that they put into it specifically, but it will take into account the granularity of the data, probably the day of the week and time of the day... then based on the history of your data collection, and throwing out some of the outliers in the historical trends so as to not be heavily swayed by other anomalies (probably using some standard deviation rules), it determines what it believes to be a accurate representation of your data and compares the current data against it....
Whether or not you feel that the data is truly outside of expectations is another story entirely
If you are using the Anomaly Detection in Workspace, this shows you where it believes the data should be, and flags items that fall outside of it's range... personally (at least in my reports) I have never felt these to be terribly accurate or useful (but I work in News... so it's a pretty volatile market, depending on what is happening... big things happening in politics? "Feel Good" community stories that go viral? Product recall notices? There are so many factors that impact what and when people are reading, that Adobe's anomalies don't really help us out much)
Here is some more reading on Anomaly Detection: https://experienceleague.adobe.com/docs/analytics-platform/using/cja-workspace/virtual-analyst/anoma... which is what is also used to create the type of alert you are seeing....
