I noticed that somone around here has a 6' wood stockade fence.

A few years after it was erected, the pickets had shrunk widthwise so
that there is a crack between every two pickets and one can see into his
yard.

Does this mean the fence builder used wood that had not aged
sufficiently when the pickets were cut?

What woods are commonly used to make pickets? Pine? Would pine
show sap? This wood doesn't have any sap or dried sap.

Thanks