Plant tags are only guides.......not something written in concrete :-) The species is reported as growing 2-6' tall so no reason why a cultivated form will necessarily stay smaller. This is particularly true with trees and shrubs but can apply to perennials and annuals as well.

I have a half dozen Iron Butterfly vernonia and mine are all about three feet tall. Their seedlings are five feet tall. I wonder if you've had some hanky panky going on. Note: my seedlings have coarser foliage than their mommas.

I think the tags underestimate the eventual height of the plants on purpose, so more shoppers will think “ooooh, this will fit into my city-lot sized garden” and then BOOM, what they thought was a 3 ft shrub rose is 5 ft tall and wide and still growing. It’s done to sell as many plants as possible to as many people as possible. That’s just my opinion, though.

They must be very happy!

Happy

Good soil fertility and moisture will push plants along to develop to their upper most maximum.