Since you have checked "On a fast network, connect using HTTP first, and then by using the TCP/IP", Outlook will connect to the server via HTTP by default when on the LAN. Unless http connection is unavailable, it will not convert to TCP/IP connection.
Fast connection defined as a connection that is faster than 128 kilobits per second
More Info Here
http://technet.microsoft.com/en-us/library/aa996069(EXCHG.65).aspx