question about high PP and overcard flops

[PokerFox]

I think it's because it's 5 am and I'm tired, but I really don't understand this:

In SSHE, it's stated that mid-high PP (especially 99 and TT) hold up amazingly well when you PF raise, even when overcards hit the flop, even with callers to the river.

Why is this? I really cannot figure it out, but have found it to be true in the last 1000 hands or so I've really started paying attention.

Cheers,
fox