Same batch of French macaroons, so why do they look so different? There are two differences, and one must hold the key to success.
I piped the dough onto two cookie sheets, and then put one sheet in the oven at a time. The macaroon on the left went into the oven first, and before putting in the oven I rapped the sheet firmly on the counter probably 15 times, in an effort to get out air bubbles that would burst through and ruin the smooth, pretty top.
The one on the right sat on the counter while the first pan was in the oven, and just before putting it in the oven, I lightly tapped the second sheet on the counter.
Based on this, it seems reasonable to suppose that the key is either letting the cookies sit before baking for 10 or so minutes, or lightly tapping instead of firmly rapping the tray. After I experiment with both variables I'll post again with a definitive answer. Although I've learned that French macaroons are so finicky that there may not be a definitive answer.