|To expand on what OtheHill said; A CRT monitor has an infinitely variable pixel size limited only by the maximum scan frequencies the monitor can handle. An LCD has a fixed pixel size determined by the number of thin film transistors built into the screen.|
Consequently, resolutions outside the native resolution can only be achieved by extrapolation. Some LCDs are better at extrapolation than others. Some extrapolated resolutions look better in relation to the native resolution than others.
So it appears your monitor cannot do extrapolation when in DVI mode and why should it. Why use a high quality digital video system then degrade it with extrapolation.
In fact I would be surprised if any LCD monitor will do extrapolation while in DVI mode and is probably more difficult to do, it defeats the whole object of DVI. Extrapolating an analogue signal during the analogue to digital conversion stage is relatively easy.