NCO phase drift

You don't need matlab to see, that accumulating the said tuning word of 25769804 with an input frequency of 10e3 results in an exact output frequency of 60. No "rounding" is involved.

10000*25769804 = 60*2**32

The reported problem is apparently related to a frequency deviation of the crystal respectively mains frequency fluctuations.

F.Y.I.: http://en.wikipedia.org/wiki/mains_frequency#long-term_stability_and_clock_synchronization

P.S.: kaz is right about a small rounding. I have been fooled by my pocket calculator. In fact the tuning word is high by about 10 ppb. At least 3 orders of magnitude below expectable crystal accuracy.

The exact tuning word would be 25769803.78