Most of the previous comparisons of formal analyses of security protocols have concentrated on the tabulation of attacks found or missed. More recent investigations suggest that such cursory comparisons can be misleading. The original context of a protocol as well as the operating assumptions of the analyst have to be taken into account before conducting comparative evaluations of different analyses of a protocol. In this paper, we present four analyses of the Zhou-Gollmann non-repudiation protocol and trace the differences in the results of the four analyses to the differences in the assumed contexts. This shows that even contemporary analyses may unknowingly deviate from a protocol's original context.