From Spotify's perspective, there is a low incentive to offer unpopular songs as 320kbps streams.
If there is only a small number of peers in the network that have the high Bitrate stream (paying users with HQ enabled on desktop clients and with interest in unpopular song X), they benefits of the peer to peer networking are less likely to pay off.
However, that does not explain why they don't offer HQ for some of their more prestigious releases, though I wouldn't be surprised if the labels hand them 'shitty' 192kbps mp3s every now and then.
After looking at the spreadsheet, I’d wager that there is a correlation between lower popularity of tracks and being 160 kbps only. The only track out of the last 40 is Michael Jackson's This is It, and a quick look up in the Spotify client gives most of them a very low 'popularity' measure.
If there is only a small number of peers in the network that have the high Bitrate stream (paying users with HQ enabled on desktop clients and with interest in unpopular song X), they benefits of the peer to peer networking are less likely to pay off.
However, that does not explain why they don't offer HQ for some of their more prestigious releases, though I wouldn't be surprised if the labels hand them 'shitty' 192kbps mp3s every now and then.
Interesting read about some of their p2p architecture (PDF): http://www.csc.kth.se/~gkreitz/spotify-p2p10/spotify-p2p10.p...
EDIT:
After looking at the spreadsheet, I’d wager that there is a correlation between lower popularity of tracks and being 160 kbps only. The only track out of the last 40 is Michael Jackson's This is It, and a quick look up in the Spotify client gives most of them a very low 'popularity' measure.
I mean, really? http://open.spotify.com/track/7kBDTeWty0z1MXjcH9twph