The weights listed for planes vary. Is this due to the weight includes the engine that may be used?

First of all,...don't trust the weights. My experience is that your plane will always weigh more than the weight on the box. Most box weights are including the engine, but they generally use the absolute lightest engine that is recommended and everyone of course uses the absolute largest engine that is recommended.

That's funny, I've often times come out lighter than the box

Ok, the weight range is for several reasons. The obvious is variances in engine and radio system weight. However, wood, especially balsa can also vary a lot in weight. Also, some guys just use more glue than others when building, even ARFs may vary a bit in the glue useage and wood weights. So there are lots and lots of things that can affect the final RTF weight.