Bill Lees pun intended he ha good points.
 
As well as the different styles there is the question of topography. If there
is a creek or ravine nearby it is not uncommon to have an elevated structure
which would leave only the supports remains. An example from Va. has pits,
vaults on one side of the site and privies on the other.
 
Another factor could be they used "police tubs" or other wooden containers as
I have seen noted at a site where I found no "vaults".
 
At williamsburg we found shallow rectangulaholes where the pasn where seated.
 
Jim Parker