I think it's like when you pass a speed trap going ten miles or less over the speed limit (most times).  They don't care about you - they are going to spend their time on the real offenders.  If you notice human doctors do the same thing - they really don't start to mention your weight until you get past the "obese" level on the BMI.  This is the voice of bitter experience.  Part of being able to deceive myself about my weight was the fact that the doctors assured me I was fine, "Just don't go much further."

You have to score their body condition yourself.  Has someone posted that chart already?  You should be able to feel ribs easily.  Someone here, I think, once had a good point.  If a "ribby" dog sounds unreasonably thin, reach down and feel your own ribs right there under your armpits.  You should be able to feel the bones easily.  I was morbidly obese (110 pounds overweight) until several months ago and I could still feel my ribs.