We provide a comparison of accurate full potential band calculations of the Fermi surfaces areas and masses of MgB2 and ZrB2 with the de Haas-van Alphen date of Yelland et al. and Tanaka et al. respectively. The discrepancies in areas in MgB2 can be removed by a shift of σ bands downward with respect to π bands by 0.24 eV. Comparison of effective masses lead to orbit averaged electron-phonon coupling constants λσ=1.3 (both orbits), λπ=0.5. The required band shifts, which we interpret as an exchange attraction for σ states beyond local density band theory, reduces the number of holes from 0.15 to 0.11 holes per cell. This makes the occurrence of superconductivity in MgB2 a somewhat closer call than previously recognized, and increases the likelihood that additional holes can lead to an increased Tc.