IMHO it is not normal, I have driven several BMW`s and my E92 M3 is on its way for next week, but idle is idle.
I did once see it on a Honda I had in the past; reving from idle to 13-1400 and back. This was obviously a mechanical problem and had to be fixed. And it was fixed. ( as far as I can remeber it was provoked by a dirty gas valve, I don`t know if this is the correct English word for it.)
Now a E92 M3 has 8 of them, maybe a problem overthere?