Throughput from Amazon

I just downloaded an MP3 album and a couple MP3 tracks from another album from Amazon. With the first album, download speeds were about 500kB/s (over 4Mb/s). For the other two tracks, the download speed dropped to under 200kB/s. How come? I noticed it, but didn't give it much thought until receiving the e-mail confirmation from Amazon, and then it was clear. Amazon Digital Services was the seller of the album, and Sony BMG was the seller of the other tracks. They were coming from another server, one which was slower, at least in terms of the bandwidth it was able to give me at the time.