joint training of det+occ would decrease the performance of occ?

[kamiLight]

In table 14 show BEVNet-R50 (bev_tiny_occ.py) give the 36.11 iou and 17.37 miou, since the paper didn't provide the occ performance of joint training model, i training the joint BEVNet-R50 (bev_tiny_det_occ.py) by myself, and i found the occ performance has been decreased! The miou is 17.32, but iou is 30.90 (from 36.11->30.90), is this result normal (Compare to bev_tiny_occ.py and bev_tiny_det_occ.py, the occ part seems to be not changed)? or i miss something important? Thanks!