laravel5.2 使用update更新数据，为什么created_at也被更新了？

<code>public function registerStore(Requests\RegisterRequest $request){
        $user=User::user();
        $res=$user->update($request->all());
        return redirect('/');
    }</code>

<code>    protected $fillable = [
        'name', 'email', 'password', 'openid','user_id','truename','tel','card','bank_name','bank_branch'
    ];

    /**
     * The attributes excluded from the model's JSON form.
     *
     * @var array
     */
    protected $hidden = [
        'password', 'remember_token',
    ];
</code>

<code>public function registerStore(Requests\RegisterRequest $request){
        $user=User::user();
        $res=$user->update($request->all());
        return redirect('/');
    }</code>

<code>    protected $fillable = [
        'name', 'email', 'password', 'openid','user_id','truename','tel','card','bank_name','bank_branch'
    ];

    /**
     * The attributes excluded from the model's JSON form.
     *
     * @var array
     */
    protected $hidden = [
        'password', 'remember_token',
    ];
</code>

<code class="php">$table->timestamp('created_at')->useCurrent();
$table->timestamp('updated_at')->useCurrent();</code>

<code class="php">$table->nullableTimestamps();</code>