These stems have formed "fasciations", a deformity causing many stems to fuse into one, like a ribbon.
You find this occasionally in all kinds of plants, like e.g. dandelions. Last year I saw it in Digitalis, foxglove, somewhere.
Actually I was kinda disappointed with it this year. This was started from seed year before last and just bloomed for the first time this year. I have had them before and they always looked so pretty because they would bush out kinda. But not this one!! Out of 5 plants, this is the only one that done this. I wonder if the seeds will be true seeds or if they will also form the same way?