The logic is simple. Someone definitely reported you post to the mods and did not report the others, so their off-topic replies were spared. Perhaps the reporter saw your first reply and reported it before others commented.
This can be a possible reason of course but do not forget to evalaute that how much off topic my reply was, or how much spam my reply was on that post. I have no idea based on which criteria my post was deleted. Do not forget that I was the first one who replied on that topic.
No need to feel bad about a few of your reported posts. Almost everyone here has had their posts deleted at one point.
If it was normal, I would not bother but it is not normal. My on topic replies are deleted, there is something wrong of course. It's not a few only.