I looked this up. It sounds like one of those test your IQ online questions.
All tortoises are turtles, but not all turtles are tortoises. (At Encyclopedia Britannica it says: "...why not just call all turtlelike creature 'turtle'? Because if the animal you are referring to is a tortoise, some wise guy is going to correct you every time."
(So it's like calling tigers and lions, felines... or how different breeds of dogs are still dogs. If you search in Hebrew they do have specific names for the various types. But I'm learning too, so I don't know them. Not that I know them in English either ;/ )
5 March 2019
The word bank proposed 'is coming' as well as 'is going', but this was not accepted. In my humble opinion, such so close to the sense words should either be accepted or not be proposed at well. Do you think it is to force us to precision? I am doubtful. For me it is a bug! (One of those times when the machine is wrong!)