Large difference between specced values and real values for LED strips?
The 'specs' (pasted below) indicate 12V, not 12.0V.  I'd guess that this product is made to be compatible with automotive 12V, which is usually in the 13.8 to 14.2V range.  Your measurements somewhat confirm this.  

Specs from the reseller's page:

Item Type: Strip
Place of Origin: Guangdong China (Mainland)
is_customized: Yes
Brand Name: G-Lighting
Average Life (hrs): 50000
LED Chip Brand: Edison
Warranty: 2
Wattage: 75 
Certification: CCC,CE,UL
Waterproof: Yes
Voltage: 12V 
Power Source: DC
LEDs Number/M: 60 pcs/m
Model Number: GL60-5630C
Power: 75W Per Pcs
waterproof: yes
Length: 5 m
Beam Angle(°): 120