iOS Textures Taking 33% Extra Memory

I was testing my iOS game and noticed a discrepancy between how much memory it was taking up and how much it thought it should be taking up. Eventually I narrowed down the problem to textures taking 33% more memory than I thought they should be taking.

For instance, I would think a 256x256 uncompressed 32-bit texture should take 256*256*4 bytes = 256k. However, I would notice the app's memory grow by something like 340k when allocating a 256x256 texture. It was as if the device were allocating enough memory to store the texture and all of its mipmaps, but I'm not using mip maps or asking for the space in any way.

This extra memory stood out, because it would only happen on certain devices. I noticed the extra memory when testing the game on an iPod Touch 4. However, the problem didn't occur on an iPhone 3GS, iPod 3G, or iPad 1.

The os versions on the devices are:

iPod 3G - iOS 3.1.2 (7D11) iPhone 3GS - iOS 4.3.5 (8L1) iPod 4 - iOS 4.2.1 (8C148) iPad - iOS 4.3 (8F190)

EDIT

Here's a little more info. I measure the app's memory like this

int PlatformIOS::GetProcessMemUsage()
{
    task_basic_info info;
    mach_msg_type_number_t size = sizeof( info );
    kern_return_t kerr = task_info( mach_task_self(), TASK_BASIC_INFO, (task_info_t)&info, &size );
    if ( kerr == KERN_SUCCESS ) 
        return info.resident_size;

    return 0;
}

This returns the same value that you will see in the Insturments program for Real Mem. It's actually very useful.

And here's how I allocate my textures:

bool GL3DTextureDataPiece::CreateTextureSurface(X3DInterfaceImpl *theInterface, int theWidth, int theHeight, PixelFormat theFormat, RefCount *thePalette, bool generateMipMap)
{
    glGenTextures(1,&mTexture);
    theInterface->SetCurTexture(this);

    glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,mLinearFilter?GL_LINEAR:GL_NE