Battery light came on after replacing battery. Voltage check out. What could it be

I recently replaced my battery in my 2012 gmc canyon. I wire brushed a ground wire and cleaned it up as it was rusty. I made sure everything was how it was before. My battery light turned on when I started it. So I check my battery with a voltmeter and it read 12.4 which is good. I tjem turned the truck on and checked the alternator and it was around 13.5 so that’s not bad. Does anyone have any idea why it could be on?