I think that had to do with Diana's wish to be photographed that much. So many people have claimed that she was hooked on the newspapers - she even discussed the amount of pictures taken of her on the Squidgygate-tapes.
Here's the passage from the transcript at:
THE INFAMOUS "SQUIDGYGATE" TRANSCRIPT
GILBEY: "Sorry, Darling, I'm resting it on my chin-- on my 'chinless.' Oh [sighs], I get so sort of possessive when I see all those pictures of you!! I get
so possessive, that's the least attractive aspect of me, really. I just see them, and think, 'Oh, God, if
only....!!!'" DIANA: "There aren't that many pictures, are there? There haven't been that many."
GILBEY: "
Four or
five today!"
DIANA: "Oh!"
GILBEY: "
Various magazines. So, Darling, I...."
DIANA: "I'm always smiling, aren't I?"
GILBEY: "Always!"
DIANA: "I thought that, today."
GILBEY: "I
always told you that. It's the old---what I call the 'PR package,' isn't it? As soon as you sense a camera---I think you can sense a camera at a
thousand yards!"
DIANA: "Yes."
GILBEY: "That smile comes on; and the charm comes out, and it stays there all the time..., and then it goes away again. But Darling, tell me, how was your tea party?"
End of quote.
I don't think Catherine is like that. But then - who really knows her that well?