Why isn't fglrx using the full area of one of the two monitors?

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http://askubuntu.com – I switched from the default driver to fglrx recently so a certain app would stop crashing. After a while I got my dual monitor system to work, except the right monitor (Acer) shows the whole desktop shunken about 90%. There's an unused margin of an inch or so all around. Both monitors are 23", identical for functional purposes but different brands. Both are displaying at 1920x1080 and show the entire desktop. Left monitor, which shows the whole desktop raster filling the physical LCD area, is an AOC plugged in by DVI. The right monitor is an Acer plugged in by HDMI. It's not possibl (HowTos)