Thanks for explaining, I've wondered that. By the way, smiling is OK on US passports[1]. I've taken a lot of visa and passport photos, and I have seen a requirement not to show teeth. But I've only once had a photo returned. And I really wasn't grinning in the photo.

Incidentally, that slighty-more-serious mood in Russia feels comfortable for me. Maybe because I tend to smile less than I "should" in the US.

[1] From https://travel.state.gov/content/travel/en/passports/require...

"You must have a neutral facial expression or a natural smile, with both eyes open"