The diet might offer a clue. I can guess what made something like that happen, but it would just be speculation. It is basically one big long growth spurt.
I,m going to email the owner and see if its a female,and if it is i'm going to go and get her.looks like they feed her peas,or beans
It's heartbreaking,and there are so many more like it,up until a few years ago tortoises were really expensive in the UK,Now everyone seems to be breeding them,i saw an ad a few weeks ago 2 greek hatchlings for £50,and the problem is going to get much worse in the next couple of years.
I see this odd sort of growth in Russians alot. The scutes on RTs have a tendency to stick together resulting in uneven growth like that shown in the pic. I suspect the cause is dryness in the growth areas between the scutes. When an RT's shell is well hydrated/moisturized, the growth tends to come in more evenly around each individual scute.
In this case, the scutes in the upper carapace seem to have all stuck together preventing new growth from coming in around each individual scute. Instead, the new growth came in along the bottom of the carapace. It looks odd, but I see no evidence that it effects the tort's overall health. I'd really like to see a cross section or x-ray of RT shells with these deformations to see what the bones underneath look like.
I'm going to respectfully disagree with Shelly's assessment. The dark part up on top is what the tortoise looked like when the person first brought it home. Then they power fed the tortoise, but they didn't provide the correct UV or the right kinds of food, and the tortoise grew too fast.
It's a beautiful habitat, but it is lacking the necessary ingredients for a healthy tortoise.