Just an observation from a relative noob.
The Manual correctly states:
True if a given animation is currently set. Animations are identified by their name (case insensitive)."
If an animation is stopped, it is not playing by definition, is reported by the inspector that playing is false, but "is playing" reports true because the animation is still set.
I just comment as this seems unnecessarily confusing, the event could be called 'is animation set'. A true 'is animation playing' option would be useful, but can be worked around with booleans.