Right if it's just in the garage it could be a poor neutral connection somewhere along the line assuming you've got 240/120vac running out there. On mwbc I've noticed the longer the run/smaller the gauge wire, the more pronounced this effect will be.
Once many years ago I was in a home where this effect seemed to happen throughout the house and it was very noticeable, lights getting brighter and dimmer as well pump cycled and other loads. It eventually turned out something was flaky in the pole transformer neutral so the pco replaced it and then it was fine.