The way I figure it, if you're tooling along in drive in 6th gear and you decide you need to go to "sport" you're going to want to downshift. Since 5th and 6th are both overdrive, the first downshift would be to 4th. So you slap the shifter over to "sport" and she shifts down to 4th where you have some power or some engine braking, whichever you desire. I mostly drive in sport mode so when I take off I just hit the right paddle twice so I'm in "6" for normal driving, but ready to downshift whenever required by simply using the paddles.
Remember you have two readouts for gear selection. The big number is the highest gear that will be shifted automatically. The number in the little gear window 1-6 is the gear the transmission is actually using at any given time.