Unicorn vs. Phusion Passenger for Rails 3 - weird session variable caching

Unicorn vs. Phusion Passenger for Rails 3 - weird session variable caching

There is a difference between a Rails 3 app using Rails cache storage for
sessions running on Unicorn and the same app running on Phusion Passenger
that I don't understand.
A user selects a bunch of items to add to a group. Those selections are
stored in a session variable. While being added to the group (ajax call),
the user starts to select more items to be added to another group using
the same session variable. The first request, however, has already
performed a session.delete(:variable_name) but the request has not
completed.
In Unicorn: The second request gets the updated session variable.
In Phusion Passenger: The second request is still getting the old session
variable.
Any ideas?
Thanks.