Hi
In BIOS I believe the video output is only sent in analogue although you can see it through the DVI it is a limited image constrained by the BIOS. When you are using the full system resources this restriction does not apply.
The BIOS is a very small piece of code, about 8 Mb's on Gigabyte boards and an update re-writes approx 0.79 Mb of this code and this 8Mb's has to handle all of the basic in/out instructions that are needed to make your PC work. The drivers for most graphics cards are 10x this size and all they have to do is drive the GPU.
Analogue video uses less resources than digital, think about the difference between a DVD and movie film, it also needs less instructions to handle image scaling than Digital images and to write the necessary instructions to power ALL digital monitors as well as the Analogue ones would mean that something else would have to be dropped from the BIOS.
Whichever cable or adapter being used, the signal being sent is analogue not digital.