Round popcorn is just made from a different variety of popcorn, called mushroom popcorn. The “normal” kind is called butterfly popcorn, or sometimes snowflake popcorn.
Mushroom popcorn kernels are a little sturdier and easier to coat, so it’s well-suited for prepackaged popcorn.
Source: https://cooking.stackexchange.com/questions/28143/how-to-make-round-popcorn
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