define table name in laravel model

when I define table name in laravel model like

class Test extends Model
    {
        use SoftDeletes;
        protected $table="en_test";

and write in my code

DB::enableQueryLog();
Test::get();
dd(DB::getQueryLog());

I have under result

"query" => "select * from `en_test` where `en_test`.`deleted_at` is null"
    "bindings" => []
    "time" => 0.88

But when I set table name like protected $table;

class Test extends Model
{
    use SoftDeletes;
    public function __construct()
       {
           $locale =\App::getLocale();
           $this->table=$locale."_test";
       }  

and write previous code. I have

"query" => "select * from `en_test`"
    "bindings" => []
    "time" => 0.93

Why wherefa_test.deleted_atis null deleted from my query?

via Chebli Mohamed