I have two cosmos in my yard that are probably about 13 ft tall. When I try to search to see how tall they get, the tallest I have found is 6 ft. The seed packet said they would get to be 3-4 ft tall; however, these are about 13 ft!! Have you ever heard of this?
Certified GKH Gardening Expert
It is not unusual to get very tall Cosmos, they can reach 6 to 8 feet, so if they are quite happy and have a nice long growing season, 13 feet seems doable.
Perhaps you want to save the seeds of your tallest plants to grow them again.
I grow Cosmos in my garden and I do find that the first year they stay smaller and with each year of self seeding they get taller.