Yes, I think you understand correctly.
In the master template, I set the template positions layout slider and number for a SB(a) - MB - SB(b) situation as usual. When modules were published in both sidebar positions for any given page, they displayed as usual. But when no modules were published to the right side of the main content area in the sidebar, instead of collapsing and allowing the main content to enlarge to fill the right side of the template, the sidebar-a modules got flipped to the right side.
If you set Sidebar-a, Main Body, Sidebar-b but there is no module assigned to Sidebar-b for a page, the default preview will be Main Body, with Sidebar-a to the right and Sidebar-b becomes collapsed.
Hence the reason you may need a template override if you want one page with a, mb, b and another page with a, mb.
Nanci