Temperature and humidity can have a small change on point of impact. The metal barrel doesn't have to change much, just a millimeter is sufficient. The stresses of the distance between the tang and barrel fasteners, can cause an ever so slight change. That is why on long rifles the barrel tannons should be slotted slightly instead of just round holes for the pins. Not only can the temperature changes affect a difference in distance between the wood and metal attachments, humidity can cause the wood to swell and shrink.
Will it throw a ball two feet to the side? Hardly. but in a hundred yards can it mean a couple inches, yes.
No body is saying the barrels bend like slinkies, but a four foot barrel does change in length by about a half millimeter just from zero degrees to 100 degrees. Foundries figure that iron shrinks by 10% from just poured molten iron to room temperature and make their casting molds 10% larger to accommodate the shrinkage. Ever see a board curl after it has been left laying on the damp ground? From the moisture swelling the contact side. Metal can bow ever so slightly just from temperature changes between one side and another. The old narrow military barrels that were held in with barrel bands were very prone to having a different point of impact between the first cold shot and when heated either by firing or ambient temperature. My trapdoor starts out 4 inches out on the first cold shot, and as the barrel heats slowly comes to center, by about the 5th shot. Muzzle loader barrels are far less subject to such changes but slight changes do occur.