Perhaps it’s a function of the Chameleon Circuit, which (perhaps) stopped being able to change the exterior of the TARDIS, but continued to be able to alter the interior…?

In the 1980s it’s revealed that the TARDIS interior can be reconfigured pretty much at will, as the Doctor converts a portion of the TARDIS into energy during an emergency in the fourth Doctor’s final story.

And more recently, in the 2008 Children in Need special “Time Crash”, the 5th Doctor says that the 10th Doctor has “changed the desktop theme”, which also implies it’s probably about as easy as flicking a few switches.

Also note how easily the tenth Doctor repaired the damage done by the spaceship Titanic in “Voyage of the Damned”, when it crashed through the TARDIS walls.

So when it’s that easy to change, not much explanation would be needed, and would be a reflection of the Doctor’s changing personality, just as he always changes his “look” (clothing) between regenerations.