what is deep sleep mode?
The ACPI (Advanced Configuration and Power Interface) standard specifies power saving modes for differnet devices(including CPUs) in modern PC(and not only). 
Any ACPI compatible CPU has to have at least these power saving modes:


C0 - Normal execution
C1 Halt - the CPU stops executing code but it can return to normal execution instantaneously. 
C2 Stop-Clock - the CPU stops and it may return to normal execution after a waiting period. 
C3 Sleep (optional) - CPU stops and the cache is lost, this differs based on implementation (some CPUs may consider this as Deep Sleep Mode)


CPUs may have additional power saver modes, some Intel processors have up to 11 power saving modes. Your particular CPU doesn't have that last C3 Sleep mode, which is useful on laptops or when you computer/server is not going to run for long periods of time, you can't for the whole thing to boot up, you want to be energy efficient.