As this its a recurring discussion/question, why sometimes an update is rolled-out delayed to user in a specific network and often Fairphone does not mention the delay when releasing the upate, here a summary of what I understood based on previous statements of FP Employees, copy of support responses (FP and carrier) and some random grab from the internet:
Fairphone controls the software releases themselves, but before anything goes out, the software is tested jointly with various network partners. These checks focus on areas where updates can have real impact on network behaviour (e.g. VoLTE/VoWiFi, emergency-calling profiles, call routing, and stability under different network conditions).
If testing shows that a new build could cause issues for users on a particular network or in a specific country, Fairphone will hold back the update for that group while the problem is fixed. The goal is simply to avoid releasing software that might disrupt calling, data, or emergency services for certain users.
Because of this, some users may see an update later than others, depending on their network provider. When your Fairphone checks for updates the update server will offer the new build only once that network profile has completed testing without major issues, so this is based on the SIM card in your phone (or the latest SIM card that connected to a network, i.e. just disabling a SIM card and switching to Wifi doesnt work).
Some context can be found here
Fairphones official (not very detailed) statement can be found here