Differential metabolism is the most common mechanism of selectivity with modern herbicides. Individuals within a species can exhibit a wide range of heritable differences to herbicides. Sugarbeets, corn, soybeans, johnsongrass, and Canada thistle are all species in which individuals can exhibit up to a 5-10 fold difference in herbicide response. Additive, antagonism and synergism are terms used to describe the amount of activity in relation to the expected response. Herbicides such as haloxyfop and fluazifop can exhibit reduced activity when tank mixed with phenoxy herbicides, acifluorfen, and bentazon.
Dimension is dicamba with an added compound which about doubles the activity.
This is generally what you expect. Several chemicals are used to improve the tolerance of large seeded grass crops to herbicides such as EPTC, butylate, alachlor, and metolachlor. These compounds are called safeners or protectants. Some are used as seed treatments, others are prepackaged with the herbicide.