About snails in archaeological sites, I´ve been working on that subject
for some years and about your assemblage:

-I´d like to know a bit more about the actual place of the site
(altitude, geology of the surroundings, etc).
-Helicids are, often, very common and widespread in Mediterranean
environments (I´m meaning vegetation belts). If you go high in altitude
you can easily found moisture-living snails.
-Rumina decollata and Cepaea nemoralis can be considered pests and are
very common in archaeological sites (helped by human activity),
regardless general environmental conditions. I think this is the reason
why they are more abundant in your second example.
-Pomatia elegans is rather stranger: Pomatia, along with R. pura, Cepaea
nemoralis and (in less degree) H. aspersa make me think about a highly
carbonated environment.
Have you checked the presence nearby of a travertine, spring or
something alike?. If there is, maybe taphonomic reasons what "forced"
your assemblage. If there isn´t, you may have two places differenciated
by human activity.
However, have you think about the differences in the total number of
individuals?. I can give you a clear reason, but as far as I have seen
the number of individual in inverse to the degree of human activity, and
more important even, a decrease in species number occurs: this is not
your case.

I´d like to follow the development of your question. Best regards,
Ana.