Question about the COMPUTE_PRECISE_BN?

[gooners1886]

hello,
in the update_bn_stats_gpu function,
workspace.FeedBlob(
'gpu{}/'.format(i) + bn_layer + '_bn_rm',
np.array(self._meanX_dict[bn_layer], dtype=np.float32),

meanX of 200 * batch_size * num_gpu training samples is computed, then rewrite the mem of bn_layer + '_bn_rm'.
so why not use the running mean accumulated during training?
why the mean computed during COMPUTE_PRECISE_BN switch is more precise?