ipad mini 6 screen size in netflix

So on my iPad Pro 11 2018 & my iPad mini 5 & my iPad mini 6 when watching Netflix screen size is stretched across the middle of the display, with big black borders top & bottom, I get it, I am watching the movie in its correct (but far from ideal) ratio. So why when I was watching the movie PASSING yesterday, it filled the entire screen, the only movie so far, but surely this should be the norm ?