Do I really need height, width, and length to calc rates?

When reaching the API if I don't include dimensions I get back "Items should have dimensions and a weight". However when the customer is going through the checkout process and requests shipping rates, the products themselves don't have dimensions, just weight.

How are these rates coming back in Shopify just fine but, with the API I must include the dimensions? Is there a default dimension used when calculating the customer shipping data?