Strange potato phenomenon

I planted some Lady Christl earlies this year and had them all back in June. They were lovely white, creamy new potatoes. I gave the same seed potatoes to our daughter who planted them in a newly dug patch in their garden. As they planted a bit later and the ground was not fertilised they were left in the ground longer than ours and they have only recently dug them up. However what they have are nice 'red skinned' potatoes. They say they taste nice, but I found this a bit odd.

Does anyone now if this is a usual phenomenon and is it due to being longer in the ground or different soil conditions?

From same bag of seed potatoes from same supplier. 

No, she only grew the ones I gave her.
It's a mystery! Maybe it's a miracle, as they say they taste great.