Well, that's because none of these measurements  represent malnutrtition on their own. Malnutrition is a complex of things and includes not only anthropometrics but also nutritional deficiencies (of either macro or micronutrients) and other clinical symptoms. Stunting on its own doesn`t mean you are malnourished. There are short people who can be well nourished but still be shorter then others due to genetics. Being underweight does not mean you are malnourished either. There are a lot of people who are very well fed but remain below the normal BMI of 18.5, either because of their genetics or for other reasons, like athletes. In female gymnastics for instance, you can find women who are shorter then average and may also be underweight but have a good muscle mass and don't have any wasting in this regard. Also, obese people can most certainly be malnourished with an abundance of nurtitional deficiencies and lost muscle mass (wasting). Wasting also happen naturally as you get old and depends not only on your nutrition but also in your physical activity.

I think reading about each of these terms (stunting, wasting, undrweight and malnutrition) independently will make you understand all of this better. Good luck!

I agree with Solomon. Stunting, wasting and underweight are good indicators of nutritional status especially in pre-school child. They occur as a result of poor dietary lifestyle which manifest as malnutrition, frequent illness or infection, poor feeding practices and hunger occasioned by poverty and war. War gives rise to increased number of displaced persons or refugees especially children who are vulnerable to poor nutritional status. In children, stunting occurs before the child is two years old and is irreversible. It is as a result of prolonged malnourishment in a child. For children, height and weight are used but for adults, other variables such as mid-arm circumference, skin fold thickness, head circumference, head/chest ratio and hip/waist ratio. Also included are physical examination, laboratory assessment, body composition, and functional data. Poor nutritional status has serious consequences. Stunting in children could result in delayed motor development, impaired cognitive function and poor school performance. According to Feldman et al. 2009, it has been shown that persons with less than 80% expected total body protein levels were prone to  increased morbidity, and ≥10%  unintentional weight loss has been associated with adverse outcomes and prolonged sickness. In lean healthy subjects, weight loss >35%, protein loss >30%, and fat loss >70% from baseline has been associated with mortality. In adults, measurement of nutritional status has no absolute indications but it is especially important during acute illness. Nutritional and public health intervention strategies should be used to treat identified malnourished states.

Apart from what has been said above: Stunting usually depicts long term effect of chronic under nutrition. Stunting doesn't happen over night and by the time the results are known, the damage is half done. It is used for long term planing and policy decisions. Wasting helps to reveal the immediate nutrition status. A child who had severe diarrhea for two days can have his/her weight depleted immediately. It is used in emergency situation (war/ disaster etc).  During survey the time of the survey also determines the wasting status particular in Asia region, for example, during monsoon the incidence of diarrhea and dysentery is high which can alter wasting prevalence. Wasted child requires immediate intervention. Under weight is more stable than wasting and commonly used in the growth monitoring program for children under five. The advantage is, it doesn't take years to show underweight like stunting. 

Dear  Khushboo Verma,

The variation in the prevalence of these indicators is due to the population to which the measurements are taken. That is why several authors associate nutritional status with socioeconomic and demographic factors, and viceversa. It is important to understand the association and involvement of economic's factors and the access to some food that are also found related. I have built a table that show how in some cases when you do not have anthropometric measures for nutritional status, it is done through economic indicators. I leave the link if it's any response .

http://www.unicamp.br/nepa/arquivo_san/volume_13_6_2011/8-Vesga-Varela-et-al-Anemia_13-06-2011.pdf

Regards.