[hu3]

My result: 18.5 to 24.9. "Healthy Weight".

I tried adding 10kgs and it said "Overweight".

Seems ok to me.

They even warn that BMI should be used along with other indicators.

[pessimizer]

Mechanick, J. (2013). “What If” Being Overweight was Good For You? Endocrine Practice, 19(1), 166–168.

https://doi.org/10.4158/endp.19.1.50042678317gx698

The evidence that being overweight is healthier is a bit dubious, but the evidence that having a "normal" BMI is healthier than "overweight" is nonexistent.