2 moved "native speaker of American English" from comment into answer, to reduce the number of comments
source | link

As a native speaker of American English, I can't justify this, but to me, in subtly connotes a much larger park than at. A person might be at the city park, but in a national park.

I can't justify this, but to me, in subtly connotes a much larger park than at. A person might be at the city park, but in a national park.

As a native speaker of American English, I can't justify this, but to me, in subtly connotes a much larger park than at. A person might be at the city park, but in a national park.

    Post Migrated Here from english.stackexchange.com
1
source | link

I can't justify this, but to me, in subtly connotes a much larger park than at. A person might be at the city park, but in a national park.