Clearly, the tripping of the (300MVA) synchronous condenser is a rather mild and quite simple contingency (see Fig. 6a). Its mean severity is of 99MW and its standard deviation is only 38MW. Considering the standard deviation of 316MW of the post-contingency margin for this contingency (see Table 2) we observe that it is indeed much simpler to ``explain'' the severity than the post-contingency margin in this case.
Actually, given the lower bound on precision of the margin computation
itself (15MW) and taking into account the random influence of the load
distribution (up to 60MW) it would not make sense to try to further
refine this information; thus the regression model reduces here to its
most simple constant shape : 
MW.