Why do plans call for 8x8x12 sized blocks?

[jab49]

In the list of materials for a 42" oven, the oven stand materials call for 63 blocks, of which 12 are to be 8x8x12 (ie, 2/3 the length of the usual 16" blocks). These aren't mentioned in the instructions at all. Am I missing something?

Thanks

John