Uhm they didn't even try to hide that it wasn't Britney. Let me save you some time before you make another amazing discovery. Britney also doesn't sing the chorus of Early Mornin on ITZ.
This was only done on Britney Jean and not He About To Lose Me, though.
If you've listened to He About To Lose Me, you would KNOW that it's not Britney. Britney sings the chorus in a lower register.
You can listen to it clearly here:
I've listened to it many times and have always just assumed it was some vocal effect
Ester Dean sings chorus parts for Rihanna on We All Want Love, and also the "na na come on" is her in S&M. She's also very audible in Where Have You Been.
Britney has been doing it for years though so it's not a surprise really.
Toxic: Cathy Dennis
Break The Ice: Keri Hilson
Gimme More: Keri Hilson
Piece of Me: Robyn
Womanizer: K.Briscoe
Circus: Claude Kelly
Hold It Against Me: Bonnie McKee
Perfume: Sia
You can hear all of these people quite obviously on these singles. I don't think Britney's fans look to her for outstanding vocals though.
Uhm they didn't even try to hide that it wasn't Britney. Let me save you some time before you make another amazing discovery. Britney also doesn't sing the chorus of Early Mornin on ITZ.