Recently I learned that a diamond shape is used for imitation of tooltip popover arrow:
Animation was made on Suspicious voting tools page
Looks like that to make an imitation more natural the edge position of a diamond relative to the bubble of tooltip should be slightly changed to prevent exit the internal part of the diamond beyond bubble boundaries.
One more thing here is that diamond boundaries are visible when the diamond is below the bubble:
And doesn't visible if the diamond at the top:
Latter two images were taken user flagged posts page, e.g. mine own.