Could someone explain how this tile is in my borders?

[jnebbe]

None of my cities have any culture besides the capital, but it's outside the capital's borders. Wondering if it's just a bug or some feature I never knew about.

 

[Tonny]

Your capital already has 2 border pops. The tile in question is now completely surrounded by your culture. In that case the tile automatically becomes part of your culture.

 

[sylvanllewelyn]

Doesn't it take 3 border pops, 60%, for the capital to reach the red square? The screenshot says 40%.

 

[Nick723]

Yes but if a tile has your culture on all 4 sides, it is included as well

 

[qouigv93027]

Yeah, it's most likely some manner of encirclement clause.