What type of coral are u keeping. KH will decreases with time. of course u can change water with a good salt mix. IMO, 11 for kH is quite comfortable. without doing anything significant, your kH will decrease with time.
accuracy in testing is diiferent for different brand of testkit. most of us .... agar agar the colour change, we take that reading for DD testkit, the colour definition is good and they also state it is valid for certain time frame for nitrate.