Some document examiners are invloved with chemical testing of documents, while others concentrate on physical and mechanical testing. The document examiner is often a qualified expert witness, accepted in court by a judge after being examined and cross-examined by counsel. It defines the outer limits of the examination, but how far the examiner can go within those limits depends on the standards. The document examiner needs to be able to rely on the standards, and the standards may need to be accepted as evidence in court. Within the bounds of reason, there cannot be too many standards.
Every handwriting shows natural variation.
In cases where varying letter forms is an issue, if the examiner doesn't see enough standard writing, he or she will not have the information needed to form a good opinion. Typewriting cases often depend on identification of some individualizing defect of a machine. Defects can be sporadic, especially in their developmental stages. A few lines of typing or a quick run through the alphabet will not yield enough material to reveal the real character of the typing element. There are unusual cases where one or two signatures or a few typewritten lines will suffice.