We report measurements and analysis of electrical spin injection and detection in Fe/MgO/Si tunnel diodes using the three terminal Hanle method. The bias dependencies of the spin-resistance-area product and the differential-resistance-area product dVc/dJ are found to be very strong (in contrast to current models) and highly correlated. We demonstrate that within a standard tunneling model a strongly bias-dependent dVc/dJ requires that the tunneling coefficient must be strongly energy- and/or bias-dependent, which produces a corresponding strong bias dependence in the spin-resistance-area product compared to the magnitude of the actual spin accumulation properties. The data are best explained if the actual spin behavior is only weakly bias-dependent, consistent with the weakly bias-depended measured effective spin lifetime.