Why doesn't my 5050 led strip draw as much power as I expect?
I once had the same problem and could not figure it out until I measured the voltage on the other end of the strip: It was dropping whole 3V accross the entire length! 
You could even see the difference in brightness when comparing the first and last LED.

Consider the resistance of the 5 meter trace of copper. These strips are usually produced on cheap flexible PCB with the default (35um) copper layers and have very high resistance.

It is clear that these LED strips will, at least in one piece, never meet the spec printed on them.