Backface checking fails

Question

Blender 2.79b.

Simple script should return indices of camera-oriented faces.

Mesh has two rather similiar directed faces, but code returns only one:

import bpy
import bmesh
import numpy

scene = bpy.context.scene

cam_mat = scene.camera.matrix_world
cam_dir = [e[2] for e in cam_mat][:-1]

bm = bmesh.new()
bm.from_object(bpy.context.active_object, scene, deform=False)
id = [f.index for f in bm.faces if numpy.dot(f.normal, cam_dir)>0]

print(id)

Default backface culling works fine though.

Answer 1

Ended with this. Transformed geometry to world coordinates (with bmesh.ops.transform as suggested by batFINGER) and replaced single camera direction to multiple directions between camera location and faces' centers:

import bpy
import bmesh
import numpy
from mathutils import Vector

scene = bpy.context.scene

obj_mat = bpy.context.active_object.matrix_world
cam_mat = scene.camera.matrix_world
cam_loc = Vector(([e[3] for e in cam_mat][:-1]))

bm = bmesh.new()
bm.from_object(bpy.context.active_object, scene, deform=False)
bmesh.ops.transform(bm, verts=bm.verts, matrix=obj_mat) #convert to world coordinates
id = [f.index for f in bm.faces if numpy.dot(f.normal, cam_loc-f.calc_center_median())>0]

print(id)