GridHelper size – maybe poorly explained.

First of all, I have to thank all the contributors of this project for its effort. 😉

Iin the Grid Helper doc, this is what says about size:

size — The size of the grid. Default is 10.

So, I’d expected from one side to the opposite to size 10 units, and the grid would be 10×10.
BUT, what really appears to be is to measure 20 x 20 units. Am i misunderstanding or what?
There it is a jsfiddle where I made a try.

[Note: forgive my poor english]

Thanks in advance.