There are at least three factors in play here:
  1. Breathing
  2. Unit tolerances
  3. Marketing



Breathing. is how the focal length of the lens changes as you rack focus. Try it. Watch the viewfinder as you focus from the minimum to infinity: you'll see it "zoom", even on a prime. This is normal for a still lens. The focal length printed on the lens is usually based on being set to infinity focus. So, getting back to your test, breathing could come into play in two ways: A) The 17-55 at 17mm has more breathing than the28-135 at 28mm. Or B) You did not have them on the same focus point (I bet you did, but even if you didn't it likely wouldn't be significant.)


Unit tolerances. A little known fact about lenses is that focal length varies with unit production. Three copies of the same lens may each have slightly different focal lengths. Your 28-135 may have been slightly wider than normal, and/or your 17-55 slightly longer than the average.


Marketing. It's pretty common for Marketing departments to exaggerate a little bit. 17.9mm may be enough of an excuse for them to call it 17mm. 380mm is close enough to call it 400mm, etc.