How are Clopay Intellicore garage door R-values so high?
Note the fine print in the chart: "Calculated door R-Value is in accordance with DSAMA TDS-163".  Here is DSAMA TDS-163.  

From the PDF:


  Refer to Figure 1. A typical insulated door has an exterior surface, interior surface, and an insulating core
  material. Also present is the insulating effect of air on a vertical surface, termed “air film”. Each of these
  elements has a unique R-value that added together create the door section R-Value.
  Rsection = Rair films + Routside surface + Rinsulation +Rinside surface 


So the 20.4 value includes not only the 2" of foam, but the R values from the outside surface, inside surface, and some air films that are assumed not to be moving.  I expect you're correct that the foam alone would provide a value more in line with the typical 6-7 R/inch you're used to.