These figures only appear when the distribution is driven by a function. Typically, with 2 different objects to populate the first object in the list will have a -1.000 to 0.000 range, and the second a 0.00to 1.000 range, meaning that first object will apper in the black areas created by the function, and the second in the whites..If 3 objects, range will be divided by 3, and the middle range object will appear in the grey levels.
If you simply want to have more, say maple trees than birches, without the need of a particular visual pattern in the population, simply raise the presence of the maple to 1.5, and reduce the birch to 0.5