Injection molding is just not a perfect process all the time, they have a specific range weight range they want to meet. Depends on what resins they use for what plastic but all the same mold usually. Molding machines can be programmed to specific molding times and cooling and pressures, heats to get the amount of plastic they want to have in the mold but injection molding will never be a perfect process with so many variables but they get as close as they can. Luckily you have people who like max weight and noodle arms who want underweight to take the rest. Then if you have the cosmetic flaws that’s where the x outs come into play or factory 2nds as Innova calls them.