Or if we want to be slightly more pendantic, why `libfdk_aac` (not SoundCloud per se) defaultly restricts the input's frequency range when encoding.

My guess is that this default surprised SoundCloud, and that this blog post is an attempt to (1) retroactively rationalize missing this difference with a reasonable-sounding explanation, even though it doesn't make sense at 256 kbps, and (2) create something that their support folks can point at when people complain.