@blight
Both mine and my neighbours bushy one were first year ( grown by me )plants. Mine looked like periwinkles, not all that bushy but with lots of flowers and my neighbours was a lot bushier, also with lots of flowers. Interestingly I'd prepared the ground as for brassica i.e. rock hard and my neighbour had just plonked hers next to her path without a care in the world. I suspect it may have been spacing them too closely as you suggest.