There must be differences in height and perhaps in foliage and bloom color.
The "parent plant" entry is typically used for genera with hundreds or thousands of plants listed. It's an easy way to add information to many plants at one time. The Ziziphus genus has only 53 plants listed, and some of them appear to be synonyms or invalid names, so it's hardly worth composing a parent plant info sheet.
The Ziziphus zizyphus entry, for example, has to be fixed. GRIN taxonomy is kind enough to make it a synonym of Ziziphus jujuba var. jujuba, but ITIS doesn't even extend it the courtesy of calling it a synonym. It says Ziziphus zizyphus is a rejected name.
