I have two here in some of my books both of which have never been very accurate. I think dogs are alot like humans and we can tell when they will be of small frame or small boned but weight can be a different issue. Like my sister and I were both 8 and 9 lb babies and were the same size basically all through our child hoods but we changed into different sizes as we each matured.
I know for a fact there are some puppies that you can just "see" are going to stay very small or be larger due to many factors, but have found that with average weight pups up to 12 wks that it can go either way "sometimes" varying a pound or so as an adult to totally surprise us. Nature. When dealing with living things we can only use a chart as a helpful guide along with our hands on of each individual pup and it's ancestors. In a perfect world all puppies would be kept until around 6 months of age and there wouldn't be as many mis-calucations...but I realize this cannot always be done (myself included) so we give the best estimate based on our knowledge.
I look at the frame of a puppy at 8 wks first and foremost. I monitor the weight throughout the growth of each baby in the litter and then refer to a past litter if it's an identical repeat breeding along with the parents, grandparents. Aunts and uncles can also be very valuable in having current information on.
If it is a first time litter between two parents, I refer to the ancestors and play it by ear until we have a bit more maturity to base it on along with the parents, grand parents, aunts and uncles. Its very important to me to keep current information on past puppies along with having accurate information on as many of the ancestors as possible. My notes over the years have turned into my own book on weights and characteristics. And yes, it has thrown me a loop before.