The type of content, the length of content, the method of ingesting the content, the different motivations/precursors to TV watching vs online watching (since the study doesn't determine causation only correlation) lots of things.

Science doesn't work by looking at things and saying what's "obvious" or "common sense", because what's supposedly obvious or common sense is often actually false.

I'm a scientist, so I know how science works. More research is always needed.

Thinking that watching video content on a different box is any different in this context is wishful thinking.