why would there be a difference between the predicted Vital capacity of lungs and calculated on by spirometry?

why cant residual volume be deterimed by spirometry? how can this be measured?

There are nearly always differences between predicted and calculated spirometry results. One of the reasons is that the human body is different from one to another even though the two are similar in weight hight age gender etc. there are severla parameters that are and could not be included i.e. unknown illnesses heredatory factors etc.

RV could be predicted :
http://thorax.bmj.com/cgi/content/abstract/47/7/519
or estimated:
http://www3.interscience.wiley.com/cgi-bin/abstract/114077082/ABSTRACT

is there are a way that you could improve the accuracy of vital capacity measurements??