They don't all have 10 petals but they all have somewhere around 10. This particular plant has been growing in my garden for two years; I grew it from seed that I collected from the plants that grow along Hickey Run where it crosses Valley Road at Beech Spring Pond. I liked those plants, but really only grew it so I could identify it at my leisure. It was clearly a Helianthus and I remember being fairly comfortable with my identification. I think it had to do with the "eared" leaves. Or something. Oftentimes I can work and work at an identification and still not be certain. It feels when you get one.
The plant itself is nice enough; it's a medium sized Helianthus, 5-6' but it seems to attract more bees than any other sunflower Of course that's an anecdotal observation and entirely subjective, but every evening the one plant will have swarms of bees. It was like that last year too. Who knows?