I just noticed recently how many songs Jenny actually doesn't sing the choruses for. I hope others didn't notice as well. I haven't seen them playing that card... yet.
The Dance chart really has zero to do with any of the single's sales.
It is just a weekly national survey of the songs that are most popular in U.S. dance clubs. It is compiled by Billboard exclusively from playlists submitted by nightclub disc jockeys.
A nobody or a commercially unsuccessful artist can have a #1 Dance Hit [cough cough Lindsay Lohan, Michelle Williams, etc], and still have flop records and albums before and after.