[woot]

This isn't even close to my field, but I imagine it has to do with wood being made of cellulose, and cellulose being made of carbon and oxygen (among other things, I assume). Heat causes the carbon and oxygen to be released in the forms of CO and CO2, and whatever is left remains as ashes. Since ashes are high in carbon, we can assume that there is more carbon in cellulose than there is oxygen.

Ok that sounds terrible but my chemistry is lacking (a problem I hope to fix eventually).

Upon further reflection, there are a lot of mistakes in the above. The oxygen and carbon necessary to "even out" the amount needed to make CO or CO2 are readily available in the air, so the carbon remaining is probably the result of a separate reaction, of which I know nothing. Meh.