It is theoretically possible, and at least scientifically interesting to consider. Some of the material dissolved in the preboil wort is, of course, precipitated out in the hot and cold break, which results in a measurable reduction in gravity. So in a very tightly controlled commercial system, where there was very minimal evaporation, sufficiently minimal contribution to gravity from dissolved hop acids and oils (this is actually significant and measurable,) and very good break coagulation, the cast wort could potentially have a lower gravity than the sweet wort. Obviously this will never happen to us or maybe anyone; but you may, if you measure everything carefully enough, notice that your gravity does not increase quite as much as the measured change in volume would predict. But in this case, something was definitely not measured carefully enough!
(IIRC this was discussed somewhere around here a while ago, where Thirsty_Monk reported that he regularly sees little or no increase in gravity after the boil in his brewery.)