These are some of the most well maintained 20-year old birds in the active Air Force!
They cycle the "family model(s)" through the lineup due to mx overhauls on the C models on a continuous basis and also if any bird goes down during show day, they'll rotate the D model into the lineup (typically Slot, while the pilot who's jet had a squawk gets Slot's plane). A late "code 3" may force a pilot into a pre-positioned D model at the end of the runway. I've seen this happen to the Boss once, when he lost nosewheel steering during the taxi out. Other scenarios could include the new incoming pilots (for the following year) getting their backseat rides at the end of the year with the pilot leaving in his/her position, necessitating the use of the "family model" aircraft again.
Throughout the some point of each season at least, routine mx cycles on the C models mean they'll rotate the D model through the lineup (flown in Slot, while the Boss or any other wingman takes the Slot pilots airplane for the show). Other times are more coincidental...
That's the best answer I have, but as you can tell, there's a procedural method to the madness.