I've been a an amateur photographer for several years. I understand the technical side and enjoy explaining it to others. But...with my 70d, which I've owned for over a year, when I take an indoor shot in Auto mode, I cannot duplicate the results in the other modes, even though I set every parameter exactly as it is in Auto. For instance, the Auto mode shows Aperture at 3.5, shutter at 1/60 and ISO at 1600. I put those settings in for Av and get a dark photo. And it's the same for M and Tv mode. Could it be Auto Lighting Optimizer? I'm stuck! Thanks!
Auto Lighting Optimizer is one potential culprit, but it is pretty easy to rule that out. The Auto(Basic Zone) mode will default ALO to Standard, so just change your ALO setting to standard in Av and run a test.
My guess is that your issue is actually the metering mode though. The Auto(Basic Zone) mode will use Evaluative Metering. Check to make sure your also in that mode when in Av or other Creative Zone modes. See this for more info on this subject: When best to use Multi-Zone/Matrix, Spot, or Center-Weight?
The other obvious difference can be the use of flash or not, but I'd imagine you already have considered that.