Answer: Some mums are hardier than others and the hardiest are the ones that come up year after year. I think that the species mum (amber colored flowers) you planted years ago has overtaken the other colored hybrids in the bed. As the more tender plants die out, the amber colored ones grow into their spaces. There's not much you can do to stop this from happening, aside from planting new mums in the bed every 2-3 years.
Best wishes with your garden!
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