Bond lengths in naphthalene
You are assuming that all resonance structures contribute equally.  The resonance structures on the right and left are equivalent but have one aromatic ring and one non-aromatic ring.  This is in contrast to the center resonance structure, which has two aromatic rings and is more stable, thus giving the bonds between carbons 1-2; 3-4; 5-6; 7-8; and 9-10 more double bond character than the other bonds and making them shorter.