I have some pretty old java blueprints and I have notice when I try to put them in the game with the grid on it is off a half a block or so.
When I try to remodel the old blue prints with the grid on the blocks are off again about a half a block.
Yes I "could" take the grid off but it is hard to line the blocks up with the other.
I didn't notice that yet but it could be.
Or maybe you built the unity building off the grid?
either try by changing the grid size with +/-
Or get it as close as possible to the desired point, they hit ctrl button to keep the blueprint in place and use the arrow keys and pgup/dn to steer it into the correct position.
You might want to change the precision to 0.001 to get it done as good as possible