There are several reasons for this:.
Some extensions have a limited renewal period. We renew the domain in our database, then hold the order, until it is allowed, and then send the renewal request to the registry when their platform accepts it (example .co.uk)
Some extensions do not have an expiration date. In these instances, we renew in our database and then submit the order to the registry, who then simply renews the domain in their database for the next period. (example .fr)
Some extensions are not renewed in realtime. In this case, we renew the domain in our database, but submit the renewal automatically before the expiration of the domain at a preset date. (example. ru)
If you have renewed your domain with Gandi, and see the expiration date as being “correct” with us, but not with the registry when you perform a whois, it may therefore be completely normal.
We still encourage you to contact our customer support, however, since while your domain may fall under one of the above cases, it is better to be sure that there is nothing else that has caused the discrepancy.