Some differences appear to be related to the particular method used to «calibrate» the proxy data against modern instrumental records. (realclimate.org)
For example, they show that the modern instrumental record averaged only over their 14 sites captures the full Northern Hemisphere mean temperature variations remarkably well over the available (approximately 150 years) interval. (realclimate.org)
Guiot's attempt to create a regression formula against the modern instrumental record has promise for the right variables. (drtimball.com)