It's both botany and the characteristics of the soil and often water,
as well.
Shen

This all assumes that Fluoride in the tiny amounts above pose any
problem at all.  read this extensive article about Fluoride from the
pauling institute:

http://lpi.oregonstate.edu/infocenter/minerals/fluoride/#content

"The lowest dose that could trigger adverse symptoms is considered to be
5 mg/kg of body weight, with the
  lowest potentially fatal dose considered 15 mg/kg of body weight."

Compare to the levels in tea, orders of magnitude are involved here.

It's not soil contamination.  It's soil.  Fluorine salts make up a lot of
different kinds of rocks, and plants pick them up from the soil.

The world is just like that, and it's fine.  You'll see similar things with
fruit juices too, along with just about any natural product.  If you worry
about this sort of stuff you'll just make yourself sick, so relax and have
a nice cup of tea.

You might, however, want to consider drinking a better grade of green tea
so you don't have to add all that crap to make it palatable, though.
--scott