Effective number of bits of 14-bit ADC
You've been bamboozled!  

14-bit is marketing speak, and the hardware also gives you that, so they'll say you have nothing to complain about. Just above ENOB in the datasheet it gives SINAD (Signal to Noise and Distortion) numbers. That's 72 dB, and 1 bit corresponds to a 6 dB level, so that 72 dB is indeed 12 bits. The 2 lowest bits are noise.  

It's possible to retrieve data which is lower than the noise floor, but it needs very good correlation, which means it has to be very predictable.