It's not only Britney Spears. Most of the artists fail to release the best songs on the album. But I am glad, Heaven On Earth or Trip To Your HEart are not single, the probabilities of it flopping are BIG. They are just not that attractive to everyone
I Wanna go is excellent and TTWE is a great single choice, smart move allow me to say.
Unusual You would have been a better choice than Radar or Amy actually and I think Breathe On Me was set to be the 6th single of In The Zone (Followed by Outrageous, Boom Boom) but
she performed it a lot of times!