Couldn't it be explained that, as she visited Earth, UK, Gloucester, probably in the 2010s, just after she stole her Tardis and the chameleon circuit was in the perfect working order, Ruth's TARDIS searched it's databanks for disguises other TARDISs may have taken on in past visits to Earth. If Ruth's TARDIS had not come across any other disguises from this planet then it would have got the chameleon circuit to take the form of the first convenient shape the TARDIS could picture up, a red phone box, a pillar box, maybe a small electric substation that looks like it might be able to power a small lighthouse.

But no, our set of Doctors have been visiting Earth in their TARDIS since the beginnings of known life on the planet, witnessing the destruction of Scaroth's spaceship, and so much more, right up to the Ninth and Tenth Doctors frequent visits while travelling with Rose, Martha and Donna. And although other TARDISs have landed here piloted by The Master, The Monk and others wouldn't Ruth's TARDIS have taken on the disguise of one which turns up in its database again and again with the same successful design of that of a blue box? 

Its a paradox of huge timey wimey proportions but how great would it be if Ruth's Doctor's TARDIS took the form of the TARDIS she would steal to get away from The Timelords far into her own future and that's why Ruth's TARDIS looks like Thirteenth's TARDIS when its dug up?

At some point in her future she returns to Gallifrey and her Tardis is placed in repairs as the chameleon circuit needed repaired after being buried under the earth for so long. Clara knows this and as it's been mothballed in repairs for years due to its agency, convinces outlet First Doctor to steal it from.