Being a sawyer, I try as best I can to make sure my timbers are within a 1/8" tolerance. But wood can move while milling, and even though the mill and blade travel in a straight line the wood can shift in even the best of mills. This shifting can cause the timber to be of different sizes from one end to the other.
Being careful to read the log and watch for stress being relieved and making the cuts in the right order or on the right side to compensate for stress relief is an important part of being a sawyer.
Doing this correctly is important.
Some sawyers sit in a booth and don't get that close to a timber to notice that it has shifted due to stress.
Personally if your timber are out 1/2"; I'd have a serious talk with your sawyer or lumber supplier and let them know this is un-acceptable. Or find another sawmill....

Jim Rogers