Difference in leaving group ability due to variation in nucleophiles
The data which you are using to explain a groups tendency to act as a good leaving group is via a nucleophilic substitution reaction.  Nucleophilic sub reactions are affected by 3 things mainly: the nucleophilicity of the nucleophile, the strength of the bond being broken and how easy is it for the leaving group to leave.

This means if you use a better nucleophile the substitution reaction will happen faster. so in the case of your data, you are comparing leaving groups using "relative rates". So the nucleophilicity of the nucleophile will end up impacting the rate. So by looking at two different nucleophiles you are looking at each nucleophiles nucleophilicity and the leaving group.