The simple answer is it's an umbrella term, like "cancer." Cancer is found in different forms, such as: breast cancer, leukemia, testicular cancer, melanoma, etc. It's no different with Dementia, there are many forms and types.
In addition, Dementias are considered severe forms of cognitive impairment that affect at least two functions of the brain. Examples include: memory, decision making, behaviors, muscle motor skills, etc. Memory loss alone does not mean Dementia.