Why is spray foam insulation typically rated at R-6.5 per inch when XPS foam board is rated at only R-5 per inch? If it's basically the same stuff - polystyrene extruded with blower gases - why wouldn't the material produced in a factory setting (with presumably greater consistency) have at least the same R-value as the material produced and applied on-site?

Since R-6.5 is the rated R-value for polyisocyanate foam board, and because spray foam insulation is available in both polyiso and polystyrene form (with polystyrene being the most common), I wonder if manufacturers are just claiming the polyiso R-value for all of their spray foam products regardless of what their actual type is.