Weight differences between SC's??

I was lookin at http://www.nadaguides.com and noticed something. they have the 1979 911SC weight at 2300LBS and change and the 81 911SC at 2600 and change is that that much of a difference in weight in only 2 years???

A/C and some other heavier options became standard equipment, and are therefore included in the weight for the later SC, but not the earlier ones.