Where does “only” go?

Some time ago I posted about adjective order (tl;dr–in English they go right before the word they refer to) and I used a Scientific American quote to show how “only” is often misplaced, put too close to the beginning of the sentence. Here’s a quote from that same noble rag (the March 2016 issue, page 48, near the top) where once again, “only” is placed at the beginning of the phrase, but this time it’s correct:

Our interest only grew when we looked more closely because we found that the vascular endothelial cells bordering the perivascular space lacked these channels.

The authors are referring what happened to their interest, and in front of “grew” is correct. Good for them!

Some might notice that you could leave the “only” out altogether without changing the meaning, so why didn’t they? They were making a subtle emphasis, conveying the idea that the growth of their interest was unexpected, a nice touch. After all, “only” is shorter than “unexpectedly.”

PS. I think they should have put a comma after “closely.” Can you tell why?