When processing a message (type: chat) that's addressed to a full JID, while the addressed resource is not online, Openfire attempts to identify another resource to deliver the message to. It does so, by evaluating all resources that have a priority of 1 or higher.

ps://tools.ietf.org/html/rfc6121#section-8.5.3.2.1 specifies that a non-negative priority (in other words: 0 or higher) should be used.