SWA probably doesn’t know which aircraft will be flying that route. Flight crews and aircraft frequently change with a moments notice.
I usually check to see what equipment will be used but that can change too. I flew this flight http://flightaware.com/live/flight/SWA1836/history on 1/5/07. I was expecting a 733 but got an empty 737 with mostly commuting pilots and crews*. The leg before this we had a 737 Shamu http://flightaware.com/live/flight/SWA1066/history/20070105/1704Z/KSEA/KMDW which I didn’t even know they had (I thought they were all 733). The lady next to me thought it was funny they had whales painted on the overhead bins. I explained and she was excited to see the outside once we landed.
*An off duty FA travelling with her husband pitched in to help with cabin service which was quickly cut off due to sky moguls. I asked her if I could have an extra bag of peanuts and I got about 50 bags dumped on me. She was very cool and easy on the eyes!