If you recall what a bell curve looks like (high in the center, tapering to the edges), it explains weights.
If you plan for an average weight of 1.00 oz, the tail ends might be 0.99 to 1.01, and half of your customers will get less than an ounce.
Instead, shoot for an average of 1.01 ounces, which moves the tails to 1.00 and 1.02, everyone gets at least an oz, and gives you the reputation of giving full value.