Ah, yes, you are right that they advise to use single back-ticks to reference variables. However, they will still appear as italic and not as monospaced code. But of course, then it is a choice which of the two you want. And it seems that the numpy docstring standard has chosen to use `` `variable` `` for variables and `` ` `int` ` `` for eg to specify the python type. 
Although I think I would prefer the variable as code, if we follow the numpy docstring standard, you can leave it like this :-)