The nobility would have a greater than 1/16 chance of snapping because their ancestors burned lerasium. The skaa have the 1/16 chance genetically because that probablitity was present when preservation initially set it up. Elend's army was mostly Skaa so the stat that we see is the 1/16. Most skaa hadn't been forced to snap before the mists started snapping people. Yes Elend brought a few noble men, but since they were mainly already allomancers they were not included in the stat.