Currently when you produce a unit, the turns of production count upward until completion. On 1/4, I have been producing for one turn and have three to go. I will never see 4/4 because the unit is done so it is instead 0/4. This makes sense.
Vectoring however doesn't work in the same way. 4/4 in vectoring is the equivalent of 3/4 in production. I can't see any logical reason for this. As far as I can tell, its simply because on the first turn of transfer, it says 1/x instead of 0/x. Having a 3/3 would be fine if production worked that way, but its seems inconsistent at best, and confusing at worst not to have vectoring and production follow the same numbering logic.
That said, there's quite possibly a reason for this that I'm not seeing.