By default, every individual host is baselined within the "Inside Hosts" host group. Stealthwatch baselines only aggregate host behavior at the host group level for the "Outside Hosts" host group.
If you turn off "Enable baselining for Hosts in this Group", Stealthwatch will baseline the aggregate host behavior at the host group level.
I have not been able to find any documentation to back this part up but I believe if your host is a member of two different host groups who are not children or parents of each other and one of them has the default enabled setting of "Enable baselining for Hosts in this Group", that host will be individually baselined. If the host is a member of a child host group who has disabled that option, the host will not be individually baselined.