Could 1 Billion Dollars of New York City Property Tax be missing?

See http://property.brooklyncoop.org/chart1

For Tax Class 1 properties, it appears that the sum of the Market Value is approximately $384 Billion but the assessed value should be 6% of the Market Value for Class 1 properties.  That would be (384 * 6%) = $23 Billion.  However according to the New York City Department of Finance the sum of the assessed values is approximately $18 Billion.  What happened to the other $5 Billion?  Ask your representative, this means that $1 Billion in property tax is not collected every year in Tax Class 1 property alone.

This does not even consider the other property tax classes.  However, we do see that Class 2 and 4 properties do pay the vast majority of property tax based on their assessment values.

New York City Tax Class Summary