CTET Class I to V 2016 Sep Paper

A teacher of Class I asks a student to count the total number of objects in a collection of pens, erasers and sharpners. The student puts all the objects in a line and starts counting. He says that there are 2 pens, 5 erasers and 3 sharpners instead of 10 objects. In which principle/ principles of counting do you think that the student is facing difficulty?