When running a command like

execute unless function skills:chances/1in3 run scoreboard players add @s skill_carpentry 1

from inside a function the return value of the tested function is not checked. The command will be executed whether it is 0 or not. Only success is used.
The called function (skills:chances/1in3) is:

return run random roll 0..2

When this command is run in chat, it works as expected and the command is only run if the returned value is 0.