Body Fat Percentage

According to this article, the ideal body fat for healthy/normal women should be 21-24% (athlete), or 24-31% (acceptable). My weight today 59kg. Body Fat 20.9%

According to that: my body fat is 12.33kg with lean body mass of 59 minus 12.33 = 46.67kg. With my goal to achieve ideal weight, I should aim for 18% (let say) = 57kg. I think my weighing machine is broken. There's no way I have body fat of only 21%.

How to measure body fat? A link here has 5 ways to measure.
Adult body fat % = (1.20 × BMI) + (0.23 × Age) − (10.8 × sex) − 5.4
Sex = 0 for man, 1 for female
BMI = body weight (kg) divide into height (in m) square. So 59kg / 1.62m = 36.4 / 1.62m = equals 22.48