css - background-position使用疑惑。

DEMO

我看文档上是这样写的:

A percentage for the horizontal offset is relative to (width of background positioning area - width of background image). A percentage for the vertical offset is relative to (height of background positioning area - height of background image), where the size of the image is the size given by ‘background-size’.

那么为什么10%就会偏移那么多呢？
不解。