The merit in question was given to you by Welsh, a moderator. I have also received merit from him for old posts of mine.

I don't know why Welsh merits old posts as he does. It is possible he is responding to reports of a post in that thread. It is also possible he is just reading old threads, either because he is searching for threads about a particular topic, or is reading a particular person's posts/threads and reading the entire thread.

Welsh merited several posts in a short time around the time when you received the merit you are questioning, many of which were fairly old. My guess is that he was likely reading old threads and merited posts he thought were deserving of merit.