By the authority vested in me as President by the Constitution and the laws of the United States of America, including sections 121 and 12304 of title 10, United States Code, I hereby determine that it is necessary to augment the active Armed Forces of the United States for the effective conduct of Operation Atlantic…
The possibly mundane explanation is that they need more rotation in an already massive force stationed in the EU and 3k isn't a very big difference:
The U.S. rushed 20,000 more troops to Europe after Russia’s invasion, bringing the total to over 100,000 on the continent. That includes new rotations of 10,000 troops in Poland, which has emerged as a critical hub for supporting and supplying Ukraine.
https://www.politico.com/news/2023/07/13/biden-military-reservists-europe-deployment-00106271