Learning is also about the big picture. Counting the number of factoids that you pick up about specific languages isn't the same thing as gaining a broader understanding of software development, or of a particular framework or even a specific codebase.
Isn't that the mantra of BI? Be careful what you measure, because that is what you will improve.
The corollary is that you don't improve what you don't measure.
> Or is it sufficient to just know that I'm grokking more each day, and that is reflected in my work?
OP's point is, I believe, that you don't know that you're really grokking much of anything and how much it's actually reflected in your work, because you're not measuring it. This can work if you have good feedback mechanism at work, though.
I mean, this is a nice saying, but it is pretty clearly not true. Lots of things that aren't measured are improved; we have not measured my two year olds vocabulary at all, but it is certainly improving.
E.g. negating A => B yields !B => !A