Dr Woeste sent a link to an article that explains how lumber design values are assigned.
http://nfba.org/uploads/Effect_of_Variability_on_Lumber_Design_Values.pdf
This is basically an excerpt from a short course taught by Drs Woeste and Bender on wood design and gives a good overview of how and why we use the design strengths that we do and why they are being monitored and in some cases modified. Their short course on the 2012 NDS is upcoming as well Oct 10-12 at VA Tech.
If you want to dabble in design or propose changes to the building codes related to the structural use of wood and its' connections, these would be two people to meet, they are instrumental in the code development process in those areas.