The value shown next to the Not Selected event is the total length of time intervals selected in Timeline Diagram. For example, your application has three threads: Thread#1, Thread#2, and Thread#3. Profiling lasted 50 ms. You want to analyze only Thread#1 and Thread#3. After you select them in Timeline Diagram, the Events filter will look like follows:
The resulting selected time interval is the sum of the Thread#1 and Thread#3 length: 50 ms + 50 ms = 100 ms.