The major carry (01111....1 -> 10000...0) is always the
worst case for DNL when you have a weighted binary DAC
as part of the setup. This could well be real. It could also
be that there are some board level issues arising from the
logic code differences in ground current, if there is any.
There might also be some settling time component to this
if you are trying to run the ADC real fast, that point is where
everything in the circuit is being kicked. Might have a look
at the supply pin-pin, looping on 4095/4096 code.