It might be, but that's not the anecdote they were sharing?

In my limited experience (ex cub leader [mixed sex groups], church kids helper, worked with children for 15 years; kids and nieces/nephews of my own) most boys behaviour declines rapidly if you don't "run them", but most girls aren't affected in the same way.

On our scout camps (mixed sex, mixed age, for the last 20 years at least) in free time the girls _tend_ to retreat to their tents and chat and the boys _tend_ to hit the field and run around like idiots. There's obviously a lot of selection bias in these groups too, they do outdoor pursuits, rough camping, etc. -- these girls are fit and hardy and as capable at kayaking/hiking/climbing/backwoodsmanship as the boys (indeed at the top end, 14yo, the girls are often fitter and stronger as they tend to grow a little earlier than the boys do). There's probably some observer bias too, of course, and this is anecdotal (but longitudinal).

My observations of toddlers at "mums & toddlers" groups suggests the ones who run around with a pram are usually boys (though that's far from exclusive; and the mothers [it's about 1:30 men] have very strong gender bias, which might be the origin of that effect).

YMMV.

Interested in other observations, FWIW I'm in the UK.

Just wanted to highlight that I like it a lot whenever people provide context and limitations to their comments. Very measured and thus easy to take at face value. Thanks a lot!