Though the answer has already been given, I've read through the thread and even I could get confused with what a possible, more direct answer, could compare to. So, that's what I'm going to do my best in writing. Hopefully this clears up any other questions you have regarding that specific nature of having a thread locked.
Moderators typically close threads in that matter in two situations. Like it's been said, a moderator would close the thread stating that there is already an active thread if a member replied to the thread and it's been more than a month since the last reply to it. In most cases that I've come to know, that isn't the reason that the thread is locked in such a manner. The more likely reason for the thread being closed is due to the fact that a thread of that nature already exists. For example, if you made a thread in Other Voting Polls about having a fire alarm, I would lock it and respond in the nature you're inquiring seeing as how there's already a thread on the subject that's active.
Also, like SilverSmeargleSplatter has stated, sticky threads are always "active", regardless of the last post. These are usually considered to be "official" threads of that particular section. If you were to reply to a sticky thread, such as the "What are you listening to" thread in Other Entertainment and it's been a month, two months, or even a year from the last reply (granted the thread is still stickied), it's pretty much a tradition to exempt sticky threads from the revival rule, regardless, due to this. However, it does depend on the moderator of that section, of course, on whether to enforce the rule on sticky threads as well. Though it is highly unlikely a thread will last a month without a reply, it has happened in the past with threads in the Questions and Feedback forum. So don't worry about reviving sticky threads if you have something important to say.
