Saved value not exposed when editing

Upon creating some code, I noticed that on editing the computed value, the previously computed value is not saved in $node_field[0]['value'] as it should (as I've thought it should). Also, the computed value field inside $node is not exposed. I tried the following codes:

if($node_field[0]['value']){
  $node_field[0]['value'] = 99;
}else{
  $node_field[0]['value'] = 11;
}

and using $node->field_mycomputed[0]['value'] as the computed value:

if($node->field_mycomputed[0]['value']){
  $node_field[0]['value'] = 99;
}else{
  $node_field[0]['value'] = 11;
}

Everything returns 11 as the value. So clearly either I'm missing something fundamental or the code is broken...