>> X = sort(randn(1,25));
>> V = rand(25,25,25);
>> Interpval = interp3(X,X,X,V,X(10),X(5),X(1))
Interpval =
0.505428142457703
>> V(10,5,1)
ans =
0.844855674576263
>> V(5,10,1)
ans =
0.505428142457703
In other words, confusion over the fact that interp3's first coordinate is X, second coordinate is Y, whereas indexing in MATLAB has first coordinate corresponding to Y and second coordinate corresponding to X.
