The “preferred mode” comes from your display through EDID. Haiku can read that, but if you use the VESA driver without BIOS patching, it can’t do anything with it, because that video mode is not provided by the VESA BIOS in your video card. With VESA, we can only pick from a fixed list of modes decided by the VESA BIOS and video card manufacturer.
You have two options:
- Enable VESA BIOS patching in the kernel settings file if that works with your VESA BIOS. This patches (a RAM copy of) the BIOS, and allows to set any mode using the VESA driver as with other video drivers
- Get a native driver working for your videocard, then Haiku will be fully in control ov the video timings.