Southwest canceled ticket, then re-booked?

[leebee]

I received email yesterday from Southwest. I had purchased one-way fare from PWM-DAY, via BWI, using RR points. The email was to notify me that points had been removed from my RR account. Turns out that SW has changed the ticket number on my ticket, so they canceled the original ticket, redeposited the points into my RR account, then reissued the ticket under a different number and deducted the points from my RR account. Same date, flight itinerary, departure/arrival times, and RR point value. Anyone know why Southwest would do this? (FYI PWM-BWI is on WN, BWI-DAY is on AirTran).