Probably since https://gerrit.wikimedia.org/r/#/c/59797/ job handling starts an external php process by default.
This happens even if the job queue is empty.
Is this intentional? I somehow doubt starting a php interpreter just to find out it's empty won't improve performance...
I'm quite sure checking the database if there are jobs to run won't be too "expensive", and can avoid lots of more expensive PHP interpreter starts ;-)