Well thief and plunderer have similar meanings. Theft usually means stealing with the use of stealth and avoidance of violence while plundering is stealing through use of force. Usually in Latin one word is used to describe multiple forms of the same thing.
And Velociraptor doesn't necessarily mean "Quick plunderer". English is a little more complex than Latin (hard to believe I know, but to be honest it's a dead language meaning no one speaks Latin anymore) and has more words for stealing. "Raptus" means seizing. So it can be said that a Velociraptor is a "Quick Seizer", "Quick Thief" or any other word given the situation.