I have a similar problem with my sitter: She loves Tigris (sh's like Granny for him) who comes to her and purrs and does not like Filou so much because he is much more reluctant with strangers (there are visitors where he just retires under the bed or in his hammock in the upper bathroom -there are others where he jumps on shoulders -I cannot tell why).
I know that she takes care very perfectly, is very reliable and loves the cats over all very much. So I accept it -as long as I can trust her that she does the right things I think it is the same situation as with a teacher: she will always have more personal sympathies for some of her pupils and be professional enough not to let the others feel.