No coraline on astrea snails

I have noticed that every time my LFS has
new delivery of astrea or bigger "turbo" snails
they come to the store completely stripped of
the coraline algae cover on their shells...
The texture on the shell does not suggest
the coraline algae was there but just bleached
and is not longer pink - the texture suggest
that the shell is NOT covered with coraline
at all...

On the other hand, astrea snails which live
in my aquarium for a year or two have a quite
heavy and smooth coating of the pink coraline
algae. This coating occassionally turns white
when the snail decides to graze on the water
edge on the glass and keeps its shell exposed
to the air for too long and the algae dies...
But even if the algae coat is white the texture
of the shell is changed and details are not
visible: they are covered by the smooth algae.

I was thinking of a reason for this diffecence...
Are these snail collected in deeper waters where
there is not much light? Is the coraline loosing
the battle there with other kinds of algae ?
Anybody can think of any other reason ?