If you have some brown cows and some black & white cows, you can put a whole fence around them and count them, but there's a lot of cows.

You can divide the cows into groups by fencing them and counting how many browns and b&w's in each section, then add up the sections.

You could say a whole fenced in section was mostly brown, and then count all the cows in there as brown. Same with the b&w's and if your sections are mostly one color over the other, ON AVERAGE the correct number of browns and b&w's show up in the final count.

Gerrymandering is strategically moving the fences without moving the cows, so that there's a change in the number of sections counted for one color, and it doesn't accurately reflect the real color split in the final count. This is not done by mistake, it must be intentional, there are an infinite number of ways to fence up a lot into groups.

The tell-sign of Gerrymandering is fences (County lines) that follow irregular or artificial boundaries. That's exactly what's needed to skew the color counts, it can be demonstrated with natural, smooth boarders, the number of sections counted for each color will be different.

Tl;Dr irregular sections can intentionally mask statical average within statistical variance. Voroni patterns reveal more accurate data.