"It is important to note that the FAA formerly tracked incidents that
did not involve potential aircraft conflicts as surface incidents.
These incidents were not classified as “runway incursions” and were
tracked and monitored separately. Most of these events are now
considered Category C or D incursions, which are low-risk incidents
with either no conflict potential or ample time or distance to avoid a
collision. This means that the total number of runway incursion
reports increased primarily because surface incidents are now
classified as runway incursions."

http://www.faa.gov/news/fact_sheets/...m?newsId=10166

There you go. Straight from the FAA.

I shot the Seattle FSDO an e-mail. Difficult to contact them by phone.

-c