[WrongAssumption]

That’s like saying risky and riskless can be used interchangeably because the both come from the word risk.

Edit: I don’t know why I even reached for an analogy, it’s exactly like saying hundred and hundredth are interchangeable.

[montalbano]

No it's like saying biannual can mean twice a year or once every two years.

Dictionaries are descriptive, not prescriptive. Language evolves.