While it's not a select list-type field, it's still capable of storing multiple (comma-delimited) values. You're probably referring to the results you get with the plugin used. The IMDb plugin, for example, only gets the one company mentioned on the main page. All studios ("production companies") are listed on the "Company credits" page. Allmovie, on the other hand, displays all studios ("produced by") on it's main page, and these are downloaded by the Allmovie script.
Maybe the IMDb plugin can be modified to get all the studios. They seem to be consistently listed at the top of the "Company credits" page under the heading "Production Companies."
