Why is my doctor telling me to gain weight?

I m male, 15 years old, 5 9.5 and 128 lbs. My doctor is saying that I should be 160 lbs at my age. I just came home from my appointment with her. She tells me to try and get to 147 lbs in a month. I looked up my bmi and it says I am at an ideal weight for my gender, height and age. So why is my doctor telling me I... show more I m male, 15 years old, 5 9.5 and 128 lbs. My doctor is saying that I should be 160 lbs at my age. I just came home from my appointment with her. She tells me to try and get to 147 lbs in a month. I looked up my bmi and it says I am at an ideal weight for my gender, height and age. So why is my doctor telling me I need to gain weight?
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