Answer is: precipitation requires fewer Ag⁺ ions in AgBr than in AgCl.
Chemical reactions:
Ksp(KBr) = 5,3·10⁻¹³.
Ksp(KCl) = 1,8·10⁻¹⁰.
Ksp is solubility product constant. The higher the Ksp value, substance is more soluble. KBr has lower Ksp, so it is easier to form precipitant of KBr than KCl.