When sending a message to multiple smart groups you need to specify whether to send to people who match ALL the criteria you've specified or ANY. This article explains the difference (it catches most people out to begin with!).
Choose ANY if you want to send your message everybody who matches one or more of the smart groups you've chosen. This will give you a longer list of recipients
Choose ALL if you want to send only to people who appear in each and every smart group you've chosen. This will give you a shorter list of recipients