I agree with the posture thing. Are you nursing? Are your breasts larger than before? If you have more weight on top, you might be slouching forward. Also pregnancy can change the ways your muscles support your spine, making it more into an S shape and taking away inches.
If you feel like your posture is fine, I would lean to a quirk in the measurement. But if you feel different when you put shoulders back and stick your chest out, you probably do need to work on your posture. Check out http://www.clevelandclinic.org/health/health-info/docs/03... for more info about a healthy posture. You might also want to try lifting some light weights to strengthen your muscles or doing Pilates or Yoga to strengthen and focus on posture. I bet you can gain back that inch and a half and feel better all around! Take care of yourself Mama!