Selecting a specific area not working [closed]

dx, dy and dz all specify a difference in x/y/z from the start point. If you think of it as a box with two sets of coordinates, it'd be from (x,y,z) to (x+dx, y+dy, z+dz), not to (dx, dy, dz). So, currently, your command's selection box goes from (775, 58, 727) to (1549, 114, 2217).

Instead, for a box 1 deep 2 up and 9 along starting at (774, 56, 717), your command should look like this:

/testfor @p[x=774,y=56,z=717,dx=1,dy=2,dz=9]