Those letters originate from the rights holders, who have leechers in the swarm, verifying that you are actively uploading data to them. Your ISP doesnt care if you torrent, or who you torrent to. They wont originate a letter unless a rightsholder requires them to.

The rightsholder has your IP address, and the name of the file you sent them. Data for those files was sent to their leechers by your IP address, perhaps not by you, but by some machine operating on your network, or through it.

It is possible that the letter to your ISP included a list of both IP addresses belonging to several of their customers, and filenames sent from all of those customers. It is possible that the ISP sent out letters to each of the individual subscribers, and just attached the full list of files from the original complaint.