Vert accuracy better than Horiz

Hi Guys,
I did ask this on an Apple dev forum, but it is far less busy,
and no intelligible answers.

iOS is frequently estimating far greater vertical accuracy than horizontal accuracy for a position fix.
It is known that vertical accuracy is far worse than horizontal accuracy when it comes to GPS.
I understand that Location Services is not limited to GPS.

Why is iOS often reporting greater vertical than horizontal accuracy?
Cheers, Art.