MP3 file quality

Hello, I’m using cubase 8 elements and I just have a little question.
When I export a mp3 file as 320kbps, why there is always a cut-line at 16khz in frequency spectrum, which is never present in a commercial 320kbps mp3? (there is a cut-line, but at 20khz)
I tried putting a commercial 320kbps with 20khz cut-line in cubase, then export it without any touch as a same bitrate, and suprise, the 16khz cut-line appear…