The spec was updated (almost 1 year ago) to define properly
the static position of the positioned grid items:
https://drafts.csswg.org/css-grid-1/issues-wd-20150108#issue-1

But we didn't apply this change in our implementation.

The specific wording from the spec (https://drafts.csswg.org/css-grid/#static-position):
"The static position [CSS21] of an absolutely-positioned child of a grid container
 is determined as if it were the sole grid item in a grid area whose edges
 coincide with the padding edges of the grid container.
 However, if the grid container parent is also the generator of
 the absolutely positioned element’s containing block, instead use the grid area
 determined in §11.1 With a Grid Container as Containing Block."

I'm attaching a simple example of a grid with just 1 cell and with padding.
Right now we're setting the static position like if it was a regular block,
but we should follow the rules in 11.1.

You can check the example live in:
http://jsbin.com/reseqa/1/edit?html,css,output

Firefox renders this properly.
 

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static-position.html 328 bytes View Download

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	static-position-current.png 1.6 KB View Download

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	static-position-expected.png 1.3 KB View Download