Nora,
 
I believe the shell you found that looks like a right-handed lightning whelk
(Busycon perversum/contraium/sinistrum) is actually a knobbed whelk (Busycon
carica).  There are subspecies, forms and variants of this guy and some do
look like a right-handed lightning whelk.   As for why shells are typically
right-handed, no one really knows and the shells ain't talking.  Sinistral
variations are not common but they do occur and it seems certain species
never have a sinistral representative while others have them in varying
numbers or percentages.  You can find sinistral Muricidae, Volutidae,
Buccinidae, and of course Melongenidae.  I am sure there are others but I
have never really looked into this and these are just the ones that come
readily to mind.  You will probably not find a sinistral Conidae (unless it
is a fossil).  As for Cypraeidae, I don't know.
 
Welcome to the list and this great and fascinating world of shells.  What
the list has really tought me is how little I know about these critters and
there endless variety and little known mysteries.  It is really great stuff.
 
Tom Eichhorst in New Mexico, USA