I paid closer attention today, and found that it's not as simple as I thought. My current guess is that the notification appears on top if the user does not seem to be active, or the taskbar cannot be seen (for example due to fullscreen remote desktop), and pops-under with an orange taskbar icon if the user seems to be currently using the computer. Clearly the behavior is more complex than I originally assumed, and I can see why it would be confusing. (I have four different apps that have decided to notify me for a calendar appointment--Outlook 2016, Windows 10's Calendar app, Outlook for Android, and the Google Calendar app--so it's pretty much impossible for me to not notice the cacophony and I never paid close attention to how Outlook did it.)