Statistical variance in the switching probability of spin-torque switching was studied on conventional in-plane MgO-MTJs. Spin-torque switching events by a current pulse longer than inverse of the attempt frequency (typically ~ 1 nsec) are well described by the thermal activation model. This model provides the relationship between the switching probability and the current amplitude. If the switching probability is unchanged for a large number of switching events, the statistical variance in the switching probability can be obtained analytically. However, we found the statistical variance in the switching probability varies widely on measured in-plane type CoFeB/MgO/CoFeB-MTJs. Contrary to a simple expectation, the slightly negative correlation was observed between the thermal stability parameter Δ and the statistical variance in the switching probability. We also measured the external magnetic field dependence along the easy axis, and found that the external field which assists the spin-torque switching dramatically increases the statistical variance of the switching probability. This behavior is considered one of the evidence of the effect of sub-volume excitation.