Difference in leaving group ability due to variation in nucleophiles Wikipedia writes:


  There can still exist contextual differences in leaving group ability in the purest form, that is when the actual group that leaves is not affected by the reaction conditions (by protonation or Lewis acid complexation) and the departure of the leaving group occurs in the rate determining step. In the situation where other variables are held constant (nature of the alkyl electrophile, solvent, etc.), a change in nucleophile can lead to a change in the order of reactivity for leaving groups. In the case below, tosylate is the best leaving group when ethoxide is the nucleophile, but iodide and even bromide become better leaving groups in the case of the thiolate nucleophile.


Why is it so?As far as I know, leaving group ability is basically how stabilized the leaving group is when it exists as an independent species. I can't see why it depends on the nucleophile
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.