Hello red1,
There are some factors that could be at work here... The following are only suggestions...
1) according to your explanation, theoretically, 10mM of KNO3 = 10 mM of NaNO2 only if all the NO3 is reduced to NO2. Most of the time, these reactions are time dependent and change according to very small amounts of contamination. If the reduction of NO3 remains constant for an extended period of time, you have probably reached the endpoint.
2) add Zn dust to the NaNO2 solution as it may be a factor.
3) Timing.... The absorbance readings will change on the same machine with time ... restandardize frequently....
Technically, NaNO2 should not be used to standardize a KNO3 test, as there are too many variations (Na vs K, NO3 to NO2, the presence of zinc, K values of the Zn reaction, these are reasons that are immediately available).
To be safe, perform tests with the same standards and the same batch of reagents. (use KNO3 standards for measuring KNO3, use NaNO2 standards for measuring NaNO2).
If you have the exact standardized test method, I can examine the test method to gather more information.
I hope this helps,
Eugene