Check the video on the front page
Monitor match is replicating the same movement to one point on the monitor. 100% means the same movement to the edge of the monitor, 50% means the same movement to 50% of the distance between the cross hair and the monitor and so on.
When the FOV is different you can only match the movement to one specific point.
There is no such thing as 1:1 for scopes however, you have several different ways of matching them depending on your preference. Games commonly use 0%, 75% or 100% as their default match.