@alxz310, I have a custom keyboard that I have modified the firmware to do what I want. (http://winkeyless.kr/). This is not something that you can do on a typical case. But what this does show is that for some reason Spotify (and probably most Mac programs) uses Rewind and FF opposed to next and previous.
You can also use the software Karabiner to remap what the keys do. This was not an option for me cause I am using on my work computer and it is not a verified software.
Re: Next / Previous media keys don't work on 3rd Party Keyboard, Play/Pause does work. [MAC]