I noticed that too. It is "Severity" in Correlation, but "Average Severity" in display view filters. But I believe they are the same, or do the same thing.
I have a correlation rule that looks events with the Severity of 75 (which is mapped to "high"), and I can filter that by using the Average Severity field in display view.
When you query with the value of 50 in "Average Severity" field, the return should only be events with the severity of 50 in a single event. It will also return events with severity of 100,150,200,250 and so fort in aggregated events, which is normal. If the event count in an aggregated event is 10, then the aggregated severity will be 10x50=500.
Yes, that is correct.
Average Severity is for a single event while Severity is the sum of all events being aggregated.
As you can see, if there is no aggregation, Severity = Average Severity.
There is no "Severity" filter in the pane because you won't know how your data in aggregated so it makes more sense to search for the Average Severity.