That other item is probably the recovery partition and cannot be removed. And one piece of advice: You are worrying way too much about this. The operating system will take care of itself pretty well. Unlike Windows users, you don't need to do nearly as much "maintenance" functions on macOS. You have a ton of space available, don't spend your time staring and comparing one display of that space with another. About This Mac is, at the top level, a summary of the system. I would bet that in the details of a System Report you will find that smaller partition is there. But it is not something for you to worry about. It is just the system and how it is constructed.