Why does `nice` with a negative argument (e.g. `nice -15`) increment niceness?

My nice is from GNU coreutils 9.1. I observed that nice -15 is equivalent to nice -n 15:

nice               # prints 0 for me, the base niceness is 0
     nice -n  15 nice   # prints 15, this is expected
sudo nice -n -15 nice   # prints -15, this is expected
     nice    -15 nice   # prints 15

-15 is a negative number. Why does it increment the niceness in the last example above? The manual (e.g. in Debian 12) does not explain this.