Why is my dentist asking me to wear the last aligner longer than 2 weeks?

I am having the invisalign i7 treatment and was looking forward to be done with the treatment after 3.5 months like promised. Yesterday I had my appointment with the dentist after 4 trays/8 weeks. Now the dentist wants me to wear the last aligner for 5 weeks instead of only 2. I don't understand why. Her explanation was "the longer the last aligner is used, the better". I don't think this makes sense and I suppose I'll have to wear the last aligner even longer until the retainer is done?