Hello Cedric,
There is a setting for this, but by default it's disabled by default for better performance.
On the advanced tabs of K2 params you need to set Disable ordering compacting to yes.
Please note that if you have a large amount of articles (several hundred or thousands) this might affect your performance.
PS. You should backup and upgrade to 2.5.11 asap for security reasons.
JoomlaWorks Support Team
---
Please search the forum before posting a new topic :)