Here's the thing you missed: "A tree gets planted".

There's no guarantee that a tree planted to "offset" a flight will grow to any kind of maturity, let alone survive three months. Some saplings just die.

Or maybe that tree got planted in a tree plantation for timber, or wood pulp, and 20 years later, it's framing timber or newsprint.

(I am someone who does revegetation projects and have planted a large number of trees)