OK Here are a couple of "out there" thoughts and ideas since folks with more experience with this plant have proposed and explored the reasonable ones:
1) maybe this is one of those plants that benefit from pruning early on -- like mums -- to keep them shorter and more compact
2) at the time you bought them, they had been grown in containers all their lives and were only a young specimen
3) or maybe different conditions like kept very dry or allowed to dry out and stressed
3) maybe they selectively thinned so that the flower spikes would be bigger
4) maybe they fed it blooming hormones and bloom boosters
...I also want to go back to ButterflyLady's question, because my first thought was too much nitrogen
-- considering how lush your flower bed is -- looks very very lovely BTW -- there's some kind of nutrient source in there. Manure? Bagged Garden Soil that already contains fertilizer though you didn't know it?
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