what are the reasons for getting 10% difference in results between 2 similar HPLC system using same solutions and columns

Dear All,

We have tested several times same reagent and solutions in 2 different Agilent HPLC Infinity II 1260 model, and we are getting about 10% difference in final result. what is the reason behind it? 

we have changed detector lamp; cell and we passed all qualification, but still same assay tested on 2 different system shows about 10 % difference. 

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