For that matter, why did your "Captain Cook" bottlebrush (supposedly a low-growing form) reach 4 metres?
Both of these questions have the same answer...they were grown from seed. To be sure that a plant will be genetically identical to its parent (i.e. to have red flowers or to retain a low habit of growth), it must be propagated vegetatively, usually by cuttings. Because of the genetic mix within seedlings, plants grown from seed will often show a significant variation from the parent. This is why named "cultivars" must never be propagated from seed.
In the case of the "Red Flowering Gum" (Corymbia ficifolia syn. Eucalyptus ficifolia), propagation from cuttings is not practical so, unless the plant is grafted using a known red flowering clone as a scion, a purchaser can never be absolutely sure that the plant will have red flowers. Colours such as pink, orange and white (as well as red) are regularly seen in seed-grown plants.
Fortunately grafted plants of Corymbia ficifolia and other Corymbia cultivars that use C.ficifolia in their parentage are now available commercially. They are not cheap but, if you want a particular flower colour, it's the only way to be sure.
|Corymbia 'Summer Beauty' - a hybrid of Corymbia ficifolia - is available as a grafted plant. Photo: Brian Walters|