background？

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 positionin优艾设计网_Photoshop问答g 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。

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