"Delta Force" is not what it's called, either. The Army cycles through bureaucratic sounding names -- "Combat Applications Group" for example -- in order to keep it confusing. "Delta" comes from one of these names -- "Operational Detachment: Delta." This sounds like an unimportant support team for a Special Forces group. "OD: Alpha" is the main team of green berets (aka, "ODA" or "the A-Team"). They are supported by 'OD: Bravo" and "OD: Charlie" (logistics support, etc.). So "OD: Delta" sounded meaningless to an uninformed listener. By the time Chuck Norris made "Delta Force" a known name for the pinnacle of Army Special Ops, they weren't actually called that anymore in reality.