SQL Server not using all CPU cores/threads

SQL Server not using all CPU cores/threads

After upgrading our SQL Server's hardware, we noticed in the Windows Task
Manager that the SQL instance is only using half of the "threads"
available to it (see screenshot).
The server has the following hardware and software:
Windows 2008 R2 Enterprise 64bit SP1
Intel Xeon E7-4870 - 4 processors (40 cores, 80 threads)
Microsoft SQL Server 2012 Enterprise Edition (64-bit)
Running select cpu_count from sys.dm_os_sys_info returns 40.
The OS sees all 80 threads. SQL Server Standard Edition supports 4
Physical CPUs which is what we have here.
Why is only half the server's processing power being used?
We have the same hardware and software on two servers and they both
exhibit the same behavior.