AD9959: phase precision degradation?

background:
freq of drive clock: 100Mhz
DAC convert rate: 500Mhz
registers: keep FR2 default setting, phase clear relative reg/bit all are set to 0.
scope sample rate: 10GSPS, DC50 coupled.
channel 1&2: keep free running@80Mhz with CPOW being set to 0
CPOW: channel phase offset word

description:
firstly, I sample ch1&chn2 4096 point simultaneously, marked as PHASE_ORIGIN
then repeatly set the phase in a way of that,
1. write 0 to CPOW
2. then sample 4096 point of CHN1 and CHN2 simultaneously, marked as PHASE_RESET
3. randomly write 0/90/180/270 to CPOW
4. then sample 4096 point of CHN1 and CHN2 simultaneously, marked as PHASE_CHANGE
finally, doing a fft transform on the data, I work out the phase of CH1 and CH2, and it is reasonable the difference between phase_of_ch1 and phase_of_ch2 corresponding to the difference between CPOW-ch1 and CPOW-ch2.

here are the phase-difference between ch1 and ch2:
PHASE_ORIGIN:
-97.757000 -97.741469 -97.695402
it is obvously the phase diff between the tow channel is -97 and remain stable.

PHASE_RESET:
phase-diff                phase-diff sub PHASE_ORIGIN
268.834330                      366.5656
-91.242570                      6.4887
-97.773648                    -0.0424
-97.656695                    0.0746
-97.715269                     0.0160
268.820552                366.5518
268.804817               366.5361
262.374164                360.1055
268.790163               366.5215
262.239606              359.9709
262.293067              360.0244

PHASE_CHANGE:
phase-diff phase-diff sub PHASE_ORIGIN
-97.841896                             -0.1106
-1.185842                                96.5454
-97.706742                             0.0245
82.364715                              180.0960
262.296594                            360.0279
-7.723721                               90.0076
88.767826                           186.4991
262.354357                        360.0856
-187.712386                         -89.9811
-7.799809                               89.9315
172.287899                            270.0192
-97.696454                             0.0348
-277.743407                         -180.0121
262.307632                         360.0389
-277.700260                       -179.9690
-97.713310                             0.0180
82.276432                           180.0077
-97.703496                            0.0278

question:
why there are a 6 degree apperanced?
when making a great change to CPOW, such as 0-to-90, 0-to-180, the phase precision of AD9959 will get degradating?
thanks for any answer.