Unexpected result with griddedInterpolant

I'm using Matlab R2011b.

My question is, given a griddedInterpolant F, with underlying values f, and two same sized matricies r and c, (which are not necessarily the same size as f), how do I get a new matrix X, that is the same as:

X = zeros(size(r));
for i=1:size(r,1)
    for j=1:size(r,2)
        X(i,j) = F(r(i,j),c(i,j));
    end
end

Here is a minimum example that demonstrates what I've tried so far

>> f = magic(5);
>> f(5,:) = [];
>> f
    17    24     1     8    15
    23     5     7    14    16
     4     6    13    20    22
    10    12    19    21     3
>> F = griddedInterpolant(f);
>> [c,r] = meshgrid(1:5,1:4);
>> r
r =

       1     1     1     1     1
       2     2     2     2     2
       3     3     3     3     3
       4     4     4     4     4
  >> c
  c =

       1     2     3     4     5
       1     2     3     4     5
       1     2     3     4     5
       1     2     3     4     5
  >> F(r,c)

The last line gives the same as f, as I would expect. That is, r and c are a pair of matricies specifying the coordinates in each dimension that I'm trying to evaluate.

However, when I try this

>> X = F(r+0.5,c)
   20.0000       NaN       NaN       NaN       NaN
   13.5000       NaN       NaN       NaN       NaN
    7.0000       NaN       NaN       NaN       NaN
       NaN       NaN       NaN       NaN       NaN

I had expected X(4,:) = NaN and for all other elements,

X(i,j) = (X(i,j) + X(i+1,j)) / 2

Instead, I get that for the first column, and the rest are NaN. I assume that Matlab is interpreting the evaluation as number (2) listed at end of question, instead of (3).

When I try F({r,c}), I get this error.

    Error using griddedInterpolant/subsref
    Grid vector is not properly defined.

From the below documentation, I would of expected that

when size(r) == size(f), X = F(r,c)
otherwise X = F({r,c})

but this appears not to be the case.

Documentation:

Evaluation

The interpolant F can be evaluated on scattered or gridded query locations as follows:

1)    Vq = F(Xq) evaluates F at the query locations Xq to produce the corresponding values Vq. The m-by-n matrix Xq represents m points scattered in n-dimensional space. Vq is m-by-1. 

2)    Vq = F(xq1,xq2,...,xqn) specifies the query locations xqi as column vectors of length m representing m points scattered in n-dimensional space. Vq is m-by-1. 

3)    Vq = F(Xq1,Xq2,...,Xqn) specifies the query locations as n n-dimensional arrays Xqi of equal size specifying the full grid. Vq is the same size as Xqi. 

4)    Vq = F({xgq1,xgq2,...,xgqn}) specifies the query locations as a grid formed by the grid vectors xgqi. Vq is the size of grid defined by xgqi, length(xgq1)-by-length(xgq2)-by- . . . -by-length(xgqn).