d8 - composer outdated - produces a list - update doesn't update

On a D8 site, when I run the composer outdated command to check for components that need updating, composer produces a list, as follows:

php composer.phar outdated

doctrine/annotations v1.2.7 v1.3.1 Docblock Annotations Parser
doctrine/common v2.5.3 v2.7.2 Common Library for Doctrine projects
drupal/clientside_validation 1.0.0-rc1 1.0.0-rc3 Add client side validation to forms.
drupal/superfish 1.0.0-rc4 1.0.0-rc5 Adds jQuery Superfish plugin to menu blocks.
egulias/email-validator 1.2.13 2.1.1 A library for validating emails
jcalderonzumba/gastonjs v1.0.3 v1.1.0 PhantomJS API based server for webpage automation
phpunit/php-code-coverage 2.2.4 4.0.5 Library that provides collection, processing, and rendering functionality for PHP code cove...
phpunit/phpunit 4.8.31 5.7.5 The PHP Unit Testing framework.
phpunit/phpunit-mock-objects 2.3.8 3.4.3 Mock Object library for PHPUnit
sebastian/environment 1.3.8 2.0.0 Provides functionality to handle HHVM/PHP environments
sebastian/exporter 1.2.2 2.0.0 Provides the functionality to export PHP variables for visualization
sebastian/recursion-context 1.0.2 2.0.0 Provides functionality to recursively process PHP variables
sebastian/version 1.0.6 2.0.1 Library that helps with managing the version number of Git-hosted PHP projects
symfony/class-loader v2.8.16 v3.2.2 Symfony ClassLoader Component
symfony/console v2.8.16 v3.2.2 Symfony Console Component
symfony/css-selector v2.8.16 v3.2.2 Symfony CssSelector Component
symfony/debug v2.8.16 v3.2.2 Symfony Debug Component
symfony/dependency-injection v2.8.16 v3.2.2 Symfony DependencyInjection Component
symfony/event-dispatcher v2.8.16 v3.2.2 Symfony EventDispatcher Component
symfony/http-foundation v2.8.16 v3.2.2 Symfony HttpFoundation Component
symfony/http-kernel v2.8.16 v3.2.2 Symfony HttpKernel Component
symfony/process v2.8.16 v3.2.2 Symfony Process Component
symfony/psr-http-message-bridge v0.2 v1.0.0 PSR HTTP message bridge
symfony/routing v2.8.16 v3.2.2 Symfony Routing Component
symfony/serializer v2.8.16 v3.2.2 Symfony Serializer Component
symfony/translation v2.8.16 v3.2.2 Symfony Translation Component
symfony/validator v2.8.16 v3.2.2 Symfony Validator Component
symfony/yaml v2.8.16 v3.2.2 Symfony Yaml Component

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running composer update immediately after produces the following results:

php composer.phar update
Loading composer repositories with package information
Updating dependencies (including require-dev)
Nothing to install or update
Generating autoload files
> Drupal\Core\Composer\Composer::preAutoloadDump
> Drupal\Core\Composer\Composer::ensureHtaccess

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What does it mean when "outdated" indicates components that need updating, but the update command indicates that nothing needs to be updated?

What are the implications of this condition?