Sometimes a different coloured shoot can be grafted on to a plant, so you'll get diferent colours on what looks like the same plant.
This can also be done with apples - you can get different varieties on what looks like the same tree, but they are actually shoots that have been grafted :)

Also it is possible for a plant to produce what's called a 'sport' - that's a shoot with different coloured flowers from the rest of the plant.
I think this is caused by a mutation in the genes.

It happened once to a Fuchsia that I grew - the flowers were pink, but all of a sudden there appeared a shoot with red flowers - that's what's known as a 'sport' :)

I notice that the lavender flowers are semi double, while the pink and red ones are single. That would take a few different mutations, so I'm voting with the rootstock sprouting up--probably the pink flowering side. Also, if you look closely at the base of the bush, and see two main stems coming out of the ground right next to each other, it may have been two cuttings started in the pot at the growers.