When I first became serious about understanding my soil I added up all the parts per million on my soil test and found it only added up to 3% of the sample.   I then called the lab manager and asked him what was in the other 97% of my soil.  To which the reply was "I don't know". 

Even though the soil test tries to determine how much is available, knowing the total picture of what you are dealing with in your soil is very helpful.  The parent material breaks down over time through biological soil acids and the weathering process.

I believe it to be a good idea to at least once on each field take a soil sample and have it run as a manure test which has results like this soil bank test.  Better yet run a subsoil sample too.  JMHO.