Interpreting Taxlists

I finally received my Washington County, VA Taxlist microfilm from Salt Lake City. I had some problems interpreting these lists. Some of the lists have column titles with detailed names. Other taxlists have columns without names. I copied the pages with Alexander Forgey's entries and should have noticed the absence of column heading on some of the lists. There may have been headings on previous pages?
I found a nice explanation of these lists at a family website which I copied as an image (see above). I agree with this person that some of the info can be conflicting. It does appear that who's taxed  and what's taxed could change from year to year. Also I have not been able to find a list of who might be exempt? I have not been able to find any websites with early Virginia tax statutes which would help me to understand the lists better.
It was very disappointing not being able to find Andrew Forgey, husband of Eleanor Randall, in any of these lists. The 1787 taxlist seemed to point to the possibility that the male in Alexander's household was Andrew. Since the 1786 and previous lists contain the names of only males over 21 he would likely not have been old enough to be listed. The 1788 taxlist does note the number of males over 16; so there should be someone over 16 noted in Alexander's household but there is not. If indeed the 1787 list noted Andrew than possibly he left his father's household to work on another farm or was a member of the Militia. I am not sure who might have been exempt from taxation so he may have been exempt? Another possibility is the person noted was a slave belonging to Alexander's wife Anne named Jane? He was only married to Anne briefly around this time. Also so many taxlists with only one tithable per household leads me to believe there may have been some deception, and some households may not have devulged all tithables.

1794 Taxlist Alexander Forgey

Every year from 1788 to 1793 lists only one tithable in Alexander's household. In 1794 three are noted, but one year later only one is noted which is very perplexing? I came to the conclusion that Alexander G. Forgey was the son of the Alexander. He was born in July of 1779 in Virginia according to a family bible. So Alexander Forgey the younger would not have been 16 in 1793 and would not have been noted. Alexander also would not appear in the 1795 list either because he turned sixteen a few months after the April taxes were collected. I was not able to establish any connections between all these men using the taxlists.  Alexander sold his property in 1795 and does not appear on the taxlists again. It is unknown what happened to him after the sale of his property. Hopefully something will surface to clear all this up.