As I understand it, some malted grains contain an enzyme that acts upon the starches in the grains, and converts the starches into fermentable sugars.To activate the enzyme, the grains have to be raised to a certain temperature immersed in water, I believe between 150 and ~160 degrees. Steeping grains do not contain this enzyme, so do not actually add any fermentable sugar to the wort, only colors and flavors. However, if you mix in enough of the grain that does have the enzyme, the other grains will have their starches converted as well.