There's often some poor journalism involved too. If archaeologists have only come across tool X in cultures which engage in (say) year-round agriculture, they'

There's often some poor journalism involved too. If archaeologists have only come across tool X in cultures which engage in (say) year-round agriculture, they'll fairly naturally come to a weakly-held belief along the lines of "cultures which invent agriculture tend to also invent tool X soon afterwards". Then evidence comes along of hunter-gatherers with tool X, and someone trying to make a rather dry press release into something more exciting writes "archaeologists are stunned because they previously believed that hunter-gatherers were foolish primitives who couldn't possibly have invented tool X".