Bitrate doesn't necessarily correlate with audio quality, you can even perceive noise or a drop of quality in the audio of the less popular songs.
I'm seriously thinking that Google just licensed the audio they're streaming, but they didn't even care about getting proper audio files and they're simply streaming the audio their users uploaded.
In my country there was a big scandal a few years ago when a couple of radio stations started playing Billboard Hot 100 songs. I guess they couldn't source the new songs in time from reputable sources for some reason (licensing restrictions, logistical failure, no idea...).
So they simply downloaded them from local torrent trackers. The thing blew up, because a guy that was uploading and seeding the albums put his own catchy promotional jingle in the middle of most songs.
I had the same observation about Google Play Music Radio's audio quality and came on here searching for the bit rate and codec thinking it must be lower for the free radio product in order to segment and further differentiate the paid tier (many services like Spotify, Rdio, and Pandora do this). I'm using an external DAC+headphone amp. Also, note that the codec matters. AAC is supposedly twice as efficient as MP3. iTunes Radio, which streams at 256kbps AAC, sounds better to me.
I'm seriously thinking that Google just licensed the audio they're streaming, but they didn't even care about getting proper audio files and they're simply streaming the audio their users uploaded.