There are two ways you can represent BOM data: expanded representation and structured representation.
In the expanded representation, BOM items correspond to parts. The quantity column in the BOM shows the sum of all parts of a given type, no matter which assembly they belong to. If the drawing contains an external reference (xref) assembly, the parts are summed within that assembly, but are not merged with parts in the current drawing. If the drawing is attached to an external BOM, the external BOM is treated as an assembly and parts are summed within the external BOM.
In the structured representation, BOM items correspond to assemblies. When the assembly is marked with a plus sign (+) in the first column of the BOM, you can click it to reveal the parts/subassemblies under that assembly. You can collapse the assembly again by clicking the minus sign (-) in the first column (the plus sign becomes a minus sign when you expand an assembly). In the structured representation, if you attach an external BOM through a Part Reference, the external BOM is considered to be an assembly.
Balloons and parts lists display data as it is shown in the BOM dialog box. When an assembly is collapsed, the parts list contains only a single entry for that assembly and balloons label the assembly using a single label. To show the parts/subassemblies of that assembly, expand the assembly. When using the structured representation, expand or collapse assemblies as appropriate to obtain the appropriate level of detail you want to show in balloons and parts lists.