Some people will do anything except use SI units

The units don't really matter as long as you're okay with your number of kids coming out with units of square root time over length(?)

Where did you get a dinosaur, or dragon egg?

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.

Without units that's not really clear, could be depth in km

It’s an empirical formula. Engineers don’t care about unit consistency as long as it works.

You need an area modifier for normally thin undergrowth clamped to a base, where multiplying would be too powerful. So you add as a general bonus to the area