The behaviour is simply due to Opus "making stuff up" above 15.6 kHz at that bitrate. This saves many bits that can be used to make the encoded file sound better and it's hard to hear the difference because the ear isn't very sensitive at that frequency. The encoder *could* code that band better, but it would require many bits (taken from lower frequencies) and the result would likely be worse.
Thanks jmvalin, very interesting. Purely for curiousity's sake, is there an approximate scale of encoded bitrate vs. frequency that the "making stuff up" process kicks in? Not trying to rock the boat, not for MY ears at any rate, just curious. Thank you for Opus.