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We use the nonequilibrium Green function method in the ballistic limit to provide a quantitative description of the conductance of graphene p-n junctions - an important building block for graphene electronics devices. In this paper, recent experiments on graphene junctions are explained by a ballistic transport model, but only if the finite junction transition width D w is accounted for. In particular, the experimentally observed anomalous increase in the resistance asymmetry between n-n and n-p junctions under low source/drain charge density conditions is also quantitatively captured by our model. In light of the requirement for sharp junctions in applications such as electron focusing, we also examine the p-n junction conductance in the regime where D w is small and find that wave-function mismatch (so-called pseudospin) plays a major role in sharp p-n junctions.