Area would help account for a really large yard, where you may want more eggs, or for a small one, where this calculation simply has too many eggs. So, egg density per square foot (or whatever unit they wanted).

Undergrowth size to me seems like its accounting for how many eggs simply aren't found. If the grass is 6" long, you'll want more eggs because they'll not all be found.

This seems to fit especially because they're added together, which means even a yard that was just dirt, no undergrowth, you'd get eggs from area alone. There's a floor on it. If it were a separate multiple then no grass would mean no eggs.