Okay so.. not sure how this wasn't caught already, but if you select a unit that can normally be grouped, and then switch to a caravan (that apparently isn't supposed to be grouped anymore) it doesn't 'deselect' that grouping in favor of single caravans, which allows you to construct parties, groups or companies of caravans. I'm not sure if this affects the income, but it certainly affects how many population are used. The same appears to hold true for pioneers.
