glreadpixels return wrong value on different screen - objective-c

Question

So I have plugin that show differently on different machine. On my machine, when I pass in the image I generate with 32 * 32 * 32 value. with every 32 have red value go from 0 - 1. I get back correctly, every 32 pixel, the red value go from 0-1.
But same code, same program I run on my co-worker machine, it not every 32 pixels but 64 pixel the red value go from 0 - 1.
Is this problem related to screen resolution? how I can get back correct value that I pass in.
Example (first 32 pixel with red value)
Pass in
0, 8, 16 24, 32, 40, ... 248
Correct one (my machine)
0, 8, 16 24, .... 248
Wrong ( co-worker machine .
0, 4, 8, 12, 16 ... 124
Here is my code to create the texture base on Bit map
glActiveTexture(GL_TEXTURE0);
glEnable(GL_TEXTURE_RECTANGLE_ARB);
glBindTexture(GL_TEXTURE_RECTANGLE_ARB, _texID);
if (_texID) {
glDeleteTextures(1, &_texID);
_texID = 0;
}
glGenTextures(1, &_texID);
glTexParameteri(GL_TEXTURE_RECTANGLE_ARB, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glTexParameteri(GL_TEXTURE_RECTANGLE_ARB, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_RECTANGLE_ARB, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_MODULATE);
glTexImage2D(GL_TEXTURE_RECTANGLE_ARB, 0, GL_RGBA8, (int)_imageSize.width, (int)_imageSize.height, 0, GL_RGBA, GL_UNSIGNED_BYTE, _image_buffer);
glClearColor(0.0, 0.0, 0.0, 0.0);
glClear(GL_COLOR_BUFFER_BIT);
Here code to generate the bit map
[self setFrameSize:_imageSize];
GLubyte al = 255; // alpha value
double increment = (al+1) / val;
_totalByteSize = val*val*val*4;
_image_buffer = new GLubyte[_totalByteSize];
for (int i = 0; i < (_totalByteSize); i++) {
_image_buffer[i] = 0;
}
vector<GLKVector3> data;
GLubyte bl = 0;
GLubyte gr = 0;
GLubyte re = 0;
for ( int b = 0; b < val; b++) {
gr = 0;
for ( int g = 0; g < val; g++) {
re = 0;
for ( int r = 0; r < val; r++) {
int ind = r + g * val + b * val * val;
_image_buffer[ind * 4 + 0] = re;
_image_buffer[ind * 4 + 1] = gr;
_image_buffer[ind * 4 + 2] = bl;
_image_buffer[ind * 4 + 3] = al;
re+= increment; // 256 / 32 = 8
}
gr+= increment; // 256 / 32 = 8
}
bl+= increment; // 256 / 32 = 8
}
And here code to read
int totalByteSize = 32*32*32*3;
GLubyte* bitmap = new GLubyte[totalByteSize];
glReadPixels(0, 0, _imageSize.width, _imageSize.height, GL_RGB, GL_UNSIGNED_BYTE, bitmap);
[_lutData removeAllObjects];
for (int i = 0 ; i <= totalByteSize /3 ; i++) {
double val1 = (double)bitmap[i*3+0] / 256.0;
double val2 = (double)bitmap[i*3+1] / 256.0;
double val3 = (double)bitmap[i*3+2] / 256.0;
[_lutData addObject:#[#(val1), #(val2), #(val3)]];
}
Does this can cause by screen with high resolution or different setting causing it to read wrong