I'm not fully sure how it works but I think the problem is that Pandora is sending you customized songs end to end based on your decided profile for that channel. It will dump a song, play it and then buffer the next one. Pandora is being a DJ for you.
The Sirius XM app can't do that. It's streaming what's being played live on the respective Sirius channels. There isn't any ability to send something specifically to you, buffer it and play it. It's 100% duty cycle of streaming because it's just digitizing the radio stream for iPhone use. It's sending the same stream to everyone using the app on that channel. It's running data constantly at a lower data rate than the higher burst of data being used to dump a Pandora song into your phone. At least the Pandora app rests while a song is being played out.
The easiest way for you to prove it would be to put the phone in EDGE mode and listen to the GSM chatter interference created on your car stereo or portable radio. Most likely what you'll hear is a lot of data transfer with Pandora and then it will stop when it's done buffering the data while the song continues to play to the end. It will then start chattering again when the next song starts to buffer.
Next play your Sirius app and my guess is that you'll hear the data chatter constantly. It won't stop and wait for the next song like Pandora does.
Basically, the Sirius App is like Iheart Radio or Emergency Radio. They push data all the time.
Pandora is like last.fm. It buffers a song sent to only you and then starts buffering a new song as the first one finishes.
