Unless you’re a seasoned shopper—or just particularly lucky—buying shoes online can often lead you to the retailer’s “return policy” section. Though some might be dissatisfied with the way the colors or silhouettes look in IRL, many exchanges or returns are due to the wrong sizes. It’s an annoying, but pretty common, problem.

One would assume a shoe size would be universal, but it’s possible for an individual to wear an 11 in Nike, but a 10 in Adidas, and perhaps even a 9 in certain dress shoes. But why? Well, the team at Highsnobiety spoke to a number of experts to find out the reason behind the inconsistencies. And the answer came down to one design component: the last.

During the production process, material is stretched over the last, which is shaped to resemble a human foot. Highsnobiety points out that each brand has its own set of lasts that are designed for a particular activity: “basketball, running, and leisure shoes all have different requirements.”

“Sport category plays an important role in the development and shape of the last,” Adidas Originals’ Oddbjorn Stavseng, Global Senior Design Director, told the publication. “For runners, the last is created to allow for linear movements that allow the foot and toes to spread whereas football players require cleats to fit more like a glove. Lifestyle lasts vary based on the design aesthetic and original inspiration.”

You can check out Highsnobiety's Why We Wear Different Sizes in Different Shoes report in the video above. The feature also highlights other factors that determine each shoe's sizing, such as the materials used.