Hibiscus is a woody shrub. As far as I know, there's no such thing as an annual shrub...
You may be thinking about this:
Hibiscus ( /hɨˈbɪskəs/ or /haɪˈbɪskəs/) is a genus of flowering plants in the mallow family, Malvaceae. IThe genus includes both annual and perennial herbaceous plants, as well as woody shrubs and small trees. (from Wiki Hibiscus).
So Hibiscus has two meanings.. it is the name of a whole genus that includes several hundred different specieds, including some annual herbaceous (non-woody) plants. It is also the common name of a number of species of woody shrubs. That is how you are using the word when your Rose-mallow or perennial hibiscus.
It's made a bit more complicated by the fact that nurseries sometimes use "perennial hibiscus" to describe the rose mallow or rose of sharon, meaning cold hardy as opposed to tropical hibiscus. The tropical hibiscus is also perennial, woody shrub, but a tender perennial not cold hardy.
Hope this helps and doesn't just confuse you more!