The May built ones were probably pilot cars, built off line at Norwood, remember they were still building chevy IIs (Novas) at Norwood in May of 66, and it probably didn't have a VIN assigned to it at the time of its actual build. Obviously there are at least 13 of these pilot cars out there with way early assembly dates, some of them may have been held around and released for sale when actual production started in late August of 66 and the VIN was assigned at the point the decision was made to make it available to the puiblic.
Body numbers are not in numeric order on a day to day basis, they do generally increase with the VINs. GM sent Fisher orders for cars based on availbility of partsand orderes on hand. These orders were assigned the Fisher body number at that point, but Fisher was free to produce them in any order that made sence to them, like grouping of one color car together therefore the body numbers get shuffled around within each days production. The 860 body number car might have been pulled forward by the Fisher scheduler (GM scheduled 912 cars a day thru Norwood) because there was a batch of gold cars (860 is a gold car) being painted first that day, and it just happened to be the first body that made it to GM that day and it got the VIN 100001 assigned when it got there.