Exact placement of objects??

[Bradj2]

Please can someone explain why a faint gap exists between 2 objects after positioning them on exact coordinates?

E.g.
X2, 2cm squares are drawn with no line fill and positioned on top of each other (X=6cm & Y=12cm) and (X=6cm and Y=10cm).

Why the faint line between the vertical boxes?
Thanks,
Brad