Trailer hubs don't come machined balanced like automotive hubs on our cars/trucks. Adding weights on the wheel can be a hit or miss.
Also steel trailer wheels and some forged aluminum center hole and lugs are stamped at the time of process. Most stamped lug holes don't come concentric to the bead seat flange.
I've watched a old truck tire dealer/truck repair shop go through his supply of 16" trailer wheels using a engine lath/dial indicators looking for just one that didn't need several 4" long weights in one spot.
Thats a big reason commercial truck tire dealers that are savvy with non automotive wheels recommend spin the tire/wheel/hub ass'y on the trailer and measure the tire for radial and lateral run out. Something most tire shops down on the corner in town don't have much if any experience with.
And short (28"-30") stiff trailer springs are a huge damper for smoothing out of balance vibrations if any.