Why does FID change once a shapefile is uploaded?

GVH 2.7 - Once a shapefile is uploaded, FID is shifted by 1. So 0 becomes 1, 1 becomes 2, etc. Why is this? I thought the shapefile is simply being copied over, but obviously something changes once it is loaded. So far I only noticed the change in FID which is not a big issue, hopefully nothing else changes.