17-55 Measurement scale question

I'm not measurebating, but I have noticed that when I take a pic at infinity on this lens the distance scale on the lens sometimes goes to infinity, but some times will not move beyond the 10 foot mark on the scale (in other words, a few mm off the infinity mark) It's not consistent.
Is this normal...I wouldn't normally ask, (in fact I hadn't noticed) but someone took a shot thru my camera and pointed it out to me. Now I find myself peeping...
Thanks good people.