When a tooltip doesn't fit exclusively above the cursor (even though it would only be exclusively above the cursor when the tooltip is shorter than 8 pixels) it gets put below the cursor even when the tooltip already fits in its default offset to the cursor or when the tooltip already fits on screen when the bottom of the tooltip is aligned to the bottom of the screen.

The 2nd attachment is after moving the mouse cursor slightly up so that the tooltip's height would be bigger than how far below the mouse cursor is from the top of the screen.