Why 2 different spelling lists?

I differentiate my instruction in the classroom.  One of the ways I differentiate in Word Study is by matching student knowledge with an appropriate spelling list.  This is done through pre-assessment at the beginning of the week.  If students already know all the spelling words for the week during the pre-assessment, there is no point in making them practice those same words for the rest of the week.   No new knowledge will be gained by this.  Therefore, these students will be given “Spelling List 2”.  The words on List 2 has the same phonics pattern as List 1 but with more challenging words.  This will allow the students to work at a level that keeps them thinking.