When you insert something into a playlist (either a song or another playlist) the next insertion you make goes at the end of the previously inserted item(s), not at the end of the currently playing song.
Dan Zimmerman and myself had an uphill battle to convince the guys@empeg to change this...

Wow, I agree with the functionality you pressed to get. In fact I think anyone who thinks different should be rounded up and tortured. Mercilessly.

Bruno

Well, maybe not the torture but, but I agree it's a lot more intuitive to get things done as you've suggested.