Supermarket potato disease

I've noticed that several varieties of potato look perfectly fine on the outside but when you cut them in half, they're brown or black in the middle.
What causes this?

 

Could be something called potato spraint or spraing, caused by a virus.
Or it could be due to a calcium deficiency.

 

I saw a programme a while back and as usual they were blaming the weather, they said the ground was dry during harvest and the contraption they use to harvest them was causing bruising.
Could be something different but I'm seeing lots with black staining.

 

Hi

also add chilling in transport can cause the black

 

There’s a range of causes identified here:

Potato Defect Identification | AHDB