On many, the gantry (Bridge) is substantially offset from center to ensure that the actual center-line of the tool is at the center of the table travel.
Depending on the design of the Z axis mechanisms, this offset can vary greatly.
Also, the arrangement of the linear components of the axis on the face of the gantry (Y or X) can, and usually does increase the the offset.
I think a "Gantry" type is where the gantry moves and a "Bridge" type is where the so called gantry (Bridge) is stationary.
Russ