Because there is enough back pressure in the tub spout to get water to
the shower head. Thank your Congressman for legislation back in the
'80s that mandated flow restrictors. That's where the pressure comes from.

I have Fredrich Ghroe fixture in several of my baths and they require a
third passive (and hidden from view) component which I believe is called
a "dual-EL" that prevents this from happening. I don't know how others
have addressed the problem.

If this is a new phenomenon it may be that your water utility has raised
the pressure somewhat. Many are doing this to compensate for water
mains that are too small in the face of increasing demand.

RB

Chris Wacalwik wrote:
When I try to fill my bathtub, water drips out of the shower head,
even when the diverter is pushed down. I'm wondering why water would
be able to go up to the showerhead if pressure is being released by
letting water through the spout. I'm curious to know if there is a way
to fix it, or if I'll need to by a new diverter.

Chris