The border spec specifies two widths for the border area: border-image-width (http://www.w3.org/TR/css3-background/#the-border-image-width) and border-width (http://www.w3.org/TR/css3-background/#the-border-width). Currently, setting the values in -webkit-border-image after the slash set the border-width; there is no implementation of border-image-width. The border-width should change the size of the border-box, but border-image-width should not affect the size of the border box, only the drawing area available to the border-image.
Brad Kemper wrote an example of the problem this causes in the section "Padding-Box vs. Fancy Corners": http://www.bradclicks.com/cssplay/border-image/Thinking_Outside_The_Box.html
This has been fixed by adding border-image, which won't behave this way. -webkit-border-image will continue to set the border widths, though, for legacy compatibility.