Buffered Kelvin-Varley divider in AD7849
Clever arrangement.

Consider what the interpolation DAC 3 is doing.

When interpolating the first s1-s2 segment, DACs 1/2 connect by S1 and S2, DAC3 'creeps' along from Vs1 to Vs2. When it finally reaches the Vs2 end-stop, the next segment s2-s3 is brought into use, S1 opens and S3 closes. At this point, DAC3 (via S2 and A1) has the same voltage on it that it had for the first segment. The amplifier used for that connection is still A1. Even if A1 has an input offset, when DAC 3 creeps along from Vs2 to Vs3, there is no break in the nice monotonic voltage steps at Vs2. 

In a conventional buffered Kelvin Varley, A1 and A2 are switched without inverting the operation of DAC 3, A1 always goes to the 'top' of DAC 3, A2 to the 'bottom'. That means that the Vs2 tap will see the offset voltage of either A1 (when interpolating the s1-s2 segment) or A2 (when interpolating the s2-s3 segment). If there is a difference in the input offsets of A1 and A2, that will show up as a step in the output voltage. If that step is larger than the resolution of DAC3, then you will have non-monotonicity.