"Left" is a positive affirmation that says you have departed that level. "Leaving" means that you intend to leave that level when it suits you but have not necessarily departed that level. This means there is uncertainty if you have actually left or not yet left.
It's like someone asking you if you are leaving your wallet behind or have you left your wallet behind. Significant difference in meaning There should be no ambiguity in radio procedures. For decades, the term "left" a level was in AIP, until some bureaucrat decided that ICAO terminology (regardless of its obvious flaws) should be used in Australia.