Hi MUR,
We did it intentionally to help a new user create a simple 2 keyframe's animation. For a new slide with 1 keyframe this keyframe is selected by default when you open the O&A editor. So pressing "+" button adds a 2nd keyframe at the end.
If you change a cursor position, pressing + button will insert a new keyframe at a selected time position, like in previous PTE 8.