question about stock music player topic

so i have a question, and this may be an ignorant one... apologies...

so the stock music player for Note 4/3/S4/S5 (basically the same as far as im concerned)...

there is an option under the menu button to add tracks to the "now playing" list. every time i go through and add songs to the list it never plays that song(s) next. It's like it just ignores that i added songs to the now playing list.

just a little background here...

so i start up the music player and go through the "tracks" and select one to start playing. (also i keep the player on shuffle), while listening to the track i hit the menu button and select "add to now playing", it takes me to the "tracks" screen where i select 1 or more tracks and hit "done".

So now i am thinking that after it finishes the current song that it should kick over to one of the ones i just added to the list, right? WRONG... it just shuffles to something else in the "tracks" list, the "tracks" list being the 1 big list of all the tracks on the phone.

am i doing something wrong? or is this meant to be used another way or what?

any answer is appreciated.