Why is it possible to edit only some reminders by tapping the pop-up when they are issued?

[WPCentral Question]

When a reminder is issued (a pop-up window), some reminders can be editted by tapping on top of the window (reminder title) after which the "Event Details" screen opens.
This is possible only for some reminders and I am not able to track the reason.
I use the built-in Calendar app.

Nokia Lumia 925, Windows Phone 8.1 Update (8.10.14176.243), Cyan