Would you provide some details of the milling operation:

• What are the mineral(s) being floated and what is the gangue?
• What is the reagent regime?
• What are the conditioning and flotation circuit operating conditions?

Having some context provides for understanding of the question and providing meaningful answers. for example, some reagents (like oils and oily surfactants) require conditioning at higher percent solids in order to effectively cover the desired mineral surfaces.

As metal is being floated, it is removed from the flotation cell and will cause the remaining slurry to read at a lower density.

Are you certain you are not reading a density on a sample which contains air injected by the cell's agitation or pumping?

The conditioning requires a high solid concentration in order to limit the quantity of reagent you have to add, because it is less diluted. 

Also the conditioning could have a role of attrition and the high solid concentration will help to clean the surface of the grains and help the fixation of the reagent. (This is true if there is no attrition cell before the conditioning).

For flottation, you need a pulp more diluted to get a good flottation. If the concentration is too high, the performance of the flottation will be worse. 

This is confirmed by lab test. Usually the lab tests are used to check what is the best solid concentration.

As far as I know the solid concentration for conditioning isusually between 50 and 70% solid, for flottation usually between 20 and 35% solid.