Dave,
Remember, those dates are "scheduled". What has been determined is that certain odd factors actually put cars in the assembly line in a certain order. Paint was one. Like colored cars were grouped - sometimes. The same "might" apply to cars with a special engine or special T-10 aluminum transmission. The groupings are interesting, but don't put much into them. The "why" is long gone. And as a side note, a 66 GT Fastback I owned was on the line about the same time, 6R09A178590, 28C scheduled. It now lives in Australia.
Jim