Throughout these installation tips, you will come across the fact that wood is a natural product and as such will have tendencies to move, expand and contract.
For this reason, the openings pre-cut to take the window units are not designed to be a snug fit - the opening allows for the movement of both wall boards and window frames without compromising shape or wall strength.
Offering up the window centrally into its opening, ideally done by two persons should result in a gap of approximately 20-30mm each side. The top gap can, at this stage be quite wide due to the wall logs not settling into place. If you find this to be the case and the window strip won't cover your gap, cut a couple of strips of spare timber (maybe from a spare roof board) approximately 20mm thick and place these between the window and wall board raising the window by the same amount. This will then mean the window strip running along the top edge, better covers this gap.