Assessment scales for ADHD have been developed using mostly male research samples. As a result, the behaviours indicating ADHD in girls may not be what you expect.
This week, the prestigious journal The Lancet published a large study identifying objective differences in the brains of people diagnosed with attention-deficit hyperactivity disorder.
It's not necessarily the case children who are young for their year are being inappropriately diagnosed with ADHD. This is a simplistic analysis.