Storage by the pound?

We are putting our house up for sale in the next couple of months. As part of making our home as appealing to buyers as possible we are beginning to pack up our belonging now to put into storage. After an exhaustive search we finally found a reasonably priced storage unit. It surprised me that many of the places we called charge not by the size of the space you rent but rather a fee per pound. How on earth is this possible? I can't imagine what kind of weight equipment you would use to weigh some of the crazy things people put in storage. That is why we finally went with a flat fee per month unit. It feels good to finally have one task on my "to do" list accomplished.