Lumber/sawdust is made from dead heartwood, which is almost all lignocellulose, so C:N is up around 400:1 or more. Wood chips, on the other hand, are mainly from branches and contain the living part of the wood just under the bark and the living growing twig ends. Winter wood chips are even more nutritious because, trees store nutrients (pulled back from the leaves) in the twigs over winter for rapid spring growth-leafing-flowering, which is why trees can begin to leaf out before the ground warms up. Due to all the proteins in the living and stored fraction of the twigs, wood chips have C:N around 100:1 as long as the chipper isn't fed anything over 3" thick. Also, the lignin is not fully polymerized in the young branch wood (not so hard) and can be broken down more easily by fungi.