The number that you are seeing are the number used by the reply service to receive replies back from the mobile phone.
You can specifiy your own reply number when sending a message (this is the Sender's ID), however any replies will go directly to the number you have given.
For example, if you specifiy your mobile number as the sender's Id, then replies will go directly to your mobile. As replies are not going back throught the our system they will not be displayed in your account's SMS Inbox.