SMD 5050 LED current draw calculation
It is drawing less than the paper 6 amps for several reasons. 


voltage drop down the long strip.  Those internal pathways are PCB traces, not superconductors. 
cheap items of foreign manufacture are not running 100% to spec. 
actually, LED strips are sized to work safely on 14 volts, i.e. Automobile battery float voltage.  So you are not running them at full spec.