Housing Construction vs Private Fixed Investment Notes

It's more than a bit unusual to plot thousands of units and billions of dollars on the same axis, but the chart conveys the important ideas. 

Single family construction is far more important than total units under construction.

Private Residential Fixed Investment, a component of GDP follows single family with a lag that might be quite long.

Private Residential Housing Units vs Private Residential Fixed Investment 

The second chart is the same as the first except that I added a line for multi-family, 5 or more units in a single building.

Total units under construction is still rising but private residential fixed construction is falling. Why is that?