As a small creator, I am handling the fulfillment all by myself. If I have 100 kits made, for example, it is easier if I order or produce 100 pieces of each of the parts needed for the kits. To order 80 of part "A", and to produce 75 of part "B", and so on, can quickly get overwhelming for me.
Missing and incorrect parts will incur an additional cost for me to ship out replacements. As a result, I have decided to keep things simple, and from my experience from previous campaigns, keeping things simple for myself means that I can fulfill the pledges to my backers promptly and accurately. Thank you for your understanding.