Volume is a mathematical calculation and a legal term.

Think about it with cooking. A tablespoon is a tablespoon. They are measured "level". Regardless if that level measure is liquid (like water, eggs or oil), fat (like butter or shortening) or dry (like flour, sugar or salt) - it is all a "tablespoon".

Can you heap liquid? No. Can you heap flour? Yes - However, it is only a tablespoon if it is level. How would it be if, when you went to the store to buy a measuring spoon, the maker of the spoon labeled it according to how high someone might heap flour while another labeled it as to how it heaps sugar (which won't heap as high as flour) ... would people trust the measuring spoons? No real cook/chef would. They want the "level" measure.

The same holds true of a FEL bucket. It is rated as to "level" full. Not how high someone can heap various loads. For one, not all goods can be heaped the same. Secondly, it would be illegal to list the capacity above what is a level measure.

So, FEL, backhoe and excavator buckets are rated for the actual volume of a level measure as calculated by the mathematical formula for volume.

How high of whatever you want to heap it is up to you - for the measure of the buckets to trusted and legal, they must be rated for level volume ... it is that simple.