What's the difference between global var post and get_post? - wordpress

Question

I use the_posts filter to add an object to each queried post. When access the added object, I get different result by using $post or get_post.
This is the code to attach the object to posts:
add_filter( 'the_posts', 'populate_posts_obj', 10,2 );
function populate_posts_obj( $posts, $query ){
if ( !count( $posts ) || !isset($query->query['post_type']) )
return $posts;
if( in_array( $query->query['post_type'], get_valid_grade_types())){
foreach ( $posts as $post ) {
if ( $obj = new Gradebook( $post->ID ) )
$post->gradebook = $obj;
}
}
return $posts;
}
Then, access the obj via $post, sometimes get the obj, sometimes not (even when it's the same post):
function get_the_gradebook(){
global $post;
return isset($post->gradebook) ? $post->gradebook : null;
}
Access the obj via get_post(), always get the obj:
function get_the_gradebook(){
global $post;
$p = get_post($post->ID);
return isset($p->gradebook) ? $p->gradebook : null;
}
I can just use the get_post() version, but it would be useful if I know why the difference.
Additional info:
If you ask the reason I attach an obj to each post, I think WordPress may take care of the caching process at the first place. Then, other caching plugins can work on my obj as if working on standard WP posts.

Answer 1

Lets explain you with a little bit pseudo code. I am trying to be broad with my approach so that my answer is relevant to StackOverflow however I still don't know how many down votes I may be receiving for this.
The simple difference is $post is a variable and get_post() is a method that means you can expect a different output from get_post() due to several dependencies however $post will only change when you explicitly do that.
Lets assume something like this
function get_post() {
return rand(0, 5);
}
$post = get_post(); /* lets assume random
value that was generated
this time was "2" */
Now each time you call get_post() its value be keep changing however the value of $post is always 2.
Coming back to the context of wordpress, $post is set using get_post() within the Loop and corresponds to the object referring to default post ID for current URL where as get_post() will take post ID as an input and return the post object.

Answer 2

$post is what WordPress considers to be the current "post" (post/page/custom post type) and can quite often end up giving you data you didn't quite expect. This is especially true if you perform WP_Query's in your template or have a template that uses data from several "posts".
By using get_post() with the ID you want the data from, you can be assured that you are getting the data you really want.