To reproduce:
1. Create a new project
2. Attach this script to the main camera:
void OnDrawGizmos () {
Transform thisTransform = transform;
Camera thisGizmoCamera = camera;
Gizmos.matrix = thisTransform.localToWorldMatrix;
Gizmos.DrawFrustum(Vector3.zero, thisGizmoCamera.fieldOfView, thisGizmoCamera.farClipPlane, thisGizmoCamera.nearClipPlane, thisGizmoCamera.aspect);
}
3. Notice that the gizmo is ALMOST perfectly aligned with the camera, except it's top and bottom parts are not tall enough

Note is most likely not limited to just the camera