Stella (girl) and Stefani (boy) are 13 year old twin sibling…

Stella (girl) and Stefani (boy) are 13 year old twin siblings. There parent brings them to their dental appointment for a prophylaxis, exam and fluoride varnish. Both twins have good homecare with minimum plaque (biofilm), no calculus and report brushing 2x daily with an electric toothbrush. However, there non-plaque induce gingivitis. What is most likely the cause?