SPO in the HBCT?

Reference: FM-I 4-90.1.
Issue: I was a little surprised at the location of the SPO at the HBCT level and his/her role. I would have thought the SPO would have been in the BSB instead. As listed in the first few chapters of the FM-I the BSB S2/3 has no role other then internal in dealing with logistics support to other units in the HBCT. The SPO at HBCT level appears to be the coordinator of support to units with the HBCT.
Question: Could you clarify this a bit?
The SPO is in the BSB. The SPO conducts planning with the brigade S1, S4, and surgeon. But he is not assigned to the HBCT staff.