Adding variables and/or Catalog Client Scripts, Catalog UI Policies,... to a single-row variable set is merely a way to easily re-use those components in other Catalog Items. Once you've added a variable set to a Catalog Item, any scripts or policies will become part of the item as you are experiencing.  It sounds like you could wrap this script in an if block to only run it if the request type is a certain value, or null if the other Catalog Items that this variable set is used in don't have the request type variable.