But it isn't. As you lean forward to simulate drops or deep drops, it will reduce your measurement. That's because your sit bone contact points are actually closer together when you lean forward than when you're upright. Go back to the shop and talk to the mice sales person and ask how to use this info. They should (did for me) ask you how you ride and how aggressive your position is.

103 is pretty narrow. You might want to try a Specialized in 130 mm width.