Is every flower on a plant unique?
Dahlia cultivars are known to undergo somatic mutations, e.g. mutations that occur during growth that can lead to different flowers. See for example this old paper. 

So it would be very possible that your bulbs yield somatic mutations that crop up as you regrow them each year. This could be the case even if your bulb has not undergone the mutation, because the mutation possibly is occurring in the meristem of the plant as it makes new shoots or inflorescences from the bulb.

Other possibilities would include epigenetic changes which do not actually modify DNA but can possibly lead to modifications of the expression of genes. These can occur due to changes in environment, but they are a bit more difficult to demonstrate as such.