I don't know why people call it "Dwarf Firepower". The correct name is Nandina domestica 'Firepower'. It is a dwarf cultivar of Nandina domestica, but it doesn't make a lot of sense to randomly add "dwarf" to its name. The variation you see in heights is similar to the variation you will see for just about any woody plant. It will perform differently in different soils and environments, and may even be rated at different ages from different sourses.

Thank you for your quick informative reply. I planted them where I hope they will grow 3 to 4 feet. If they don't I'll move them in a few years. Ann

Many heavenly bamboos mildew in my area now.

I've have several regular sized Nandinas and so far so good on the mildew problem. These little ones will get more sun than my established ones. Ann