OpenCV SURF comparing descriptors

Question

Folowing snippet is from OpenCV find_obj.cpp which is demo for using SURF,

double
compareSURFDescriptors( const float* d1, const float* d2, double best, int length )
{
    double total_cost = 0;
    assert( length % 4 == 0 );
    int i;
    for( i = 0; i  best )
            break;
    }
    return total_cost;
}

As far as I can tell it checking the euclidian distance, what I do not understand is why is it doing it in groups of 4? Why not calculate the whole thing at once?