Alliums changing colour?

Hi all, nearly three years ago i bought 5 good sized allium cristophii bulbs and planted them in a large pot. They produced lovely heads, the second year producing 8.  So this year i have planted the bulbs (now increased in size and number to 8 good sized bulbs) in the garden. But here's the bit i don't understand .....8 alliums coming out nicely, but 3 of them are dark purple and look very much like purple sensation, the other 5 not quite so far open but definitely look like the cristophii that i planted.  Could they have changed colour? I know plants do occasionally change colour but i wasn't expecting this.

I'm sure there will be some bright spark (or sparks) out there with an explanation!