Personaly, I never make conclusions by comparing one print to one print. Or even one print to many. But if you heated a print befor it was dry, probably the viscosity plunged down and the coating spread out and formed a thiner layer of emulsion. Then you are comparing different thicknesses, There very well could be a reduction in ISO for "older" dried plates. But you would have to treat them identicly, except for aging, to make that conclusion.