It's a brain scramble because you can't just read in toto.

Well, you can literally, but, you don't feel like you grok it.

And when you take a second pass you gotta slow down and stop and parse "what's r here? is it relevant? oh rows?"

It's one of those things that's hard to describe because it doesn't sound like much if you got it. And this example is trivial, especially if you're not open to the idea its a problem (r/c = rows/columns) But it's nigh-impenetrable in other scenarios.

You feel like you're barely understanding something that you actually might grok completely.