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Yes, this is the problem with BMI. You mentioned it was an online health thing - I presume this means that you didn't see a doctor in person? If you had (and you look like the guy in that link) there's no way they would have told you you're overweight.
According to BMI someone like Chris Hoy would be overweight, so you're in good company :)
Edit: I'd like to see what happened if a doctor told Chris Hoy he was overweight actually
For men aged 20-39, the healthy range for body fat percentage runs from 8% to 20%, and for men aged 40-59 it’s 11-22%.
http://www.coachmag.co.uk/exercises/lose-weight/3809/what-is-a-healthy-body-fat-percentage
Of course being superlean but not exercising may be less healthy than a small tyre and exercising, so, it's only a factor of health.
You seem to walk/run/do sports a lot I wouldn't worry! :)