Sometimes topics are closed the day of the last reply but sometimes it is 3 months after the last reply. Why is that?
If you look at the verbiage, the ones closed after 3 months explain they were automatically closed 91 days after the last reply. This locks out old, dead threads to keep things fresh. The others were closed by moderators for various reasons, sometimes explained in the thread before the closure. Sometimes it’s because a topic was resolved, sometimes it’s because things are heading towards a flame war, etc. There are tons of reasons a mod would lock a thread.