Because even though Harmonix only officially puts songs into the in game store on Thursdays, they actually show up about 72ish hours after they pass review.
This is online at xbox.com and in the xbl store.
So if a song passes on Thursday night, then it should show up everywhere BUT the in game store around Sunday night.
NOW having said that the reason Harmonix only officially releases songs on Thursday is because once a song passes it needs to be looked over internally, and mistakes can be found, resulting in the song getting pulled, and a new version put into review.
So unless a song is officially in the in game store via HMX you run the risk of buying a song that might have an issue and get pulled.
So buyer beware.
Check out the TAG website here! :