Maybe they just re-checked their subscribers to confirm that they sent those messages to them all, and when they didn't see "thank you" message on your wall, they thought that they forgot to post it.
Probably I've committed this crime the couple of times too I mean, there were the cases when I was sure that I posted "Thank you" message, but then I've seen that there is no such post on their walls... and I don't know for sure if I forgot or they deleted it so I started to doubt what happened and what to do After that, just in case, I've sent them "Thank you" message in PM. Because, again: if I forgot to send them this message, I probably should do it; but if they deleted it, they probably don't want such a messages in their wall, and PM probably will make them less annoyed than wall post in this case
I think that they are doing it without bad intention. It means that people like your work, even if they show it in annoying way.
As far as I know, there are many non native speakers, so it is fine, don't worry!