Center frequencies must be greater than 20 Hz and less than 20,000 Hz. If the center frequency specified is not valid, it will be rounded to nearest valid value. In your case the nearest valid value is 25.12.
You can know the valid center frequency values of a specific filter using “validfrequencies” as follows:
d1 = fdesign.octave(3,'Class 1','N,F0',6,10,4096);
validcenterfreq = validfrequencies(d1);
