The more difficult question to answer is: Why is the guy with 20 Int and 8 Dex better at dodging stuff than the guy with 18 Int and 18 Dex.
or later on:
Why is the guy with 30 Int and 8 Dex better at dodging stuff than the guy with 28 Int and 28 Dex.
NWoD got it right.
Dexterity and Wits can be added to the Defense value, but only the lowest attribute is added, and not the highest like in D&D.