Corrupt Files

Hi all.
I'm confused (again)
My nephew had a problem with his HDD.
No back up and he had loads of pics.
I tried recovering files for him using Recuva
It found thousands but mostly corrupt.
The thing is, when I first scroll down the page of files, for a second I can see the pics clearly, then they break up and have lines across them.
I get they are corrupt, but how come they appear normal at first glance?
The one on the left I captured by doing a prtscn real quick
The one on the right is the same file a second later and merged with another pic?