Apparently they have at least two types of nozzles used on the UE900. When I first got my UE900, I was having lots of trouble getting balanced sound out of it, it kept sounding like the left side was louder. Upon further inspection, I noticed the eartips did not sit as far down on the left side as the right. Then, removing the eartip I noticed the flange size was different on both pieces. I also noticed the left side had an extra piece of triangular shaped plastic on the tip which the right side doesn't.
I had to replace it with another pair which now both tips match the left tip of the old pair. Weird, I wonder why the nozzle change, I wonder if one sounds better than the other.