Print Shifting

I'm trying to print off a mount for a small Raspberry Pi camera module so I can affix one to my printer for using in OctoPrint. When I print out the base that mounts to the aluminum extrusion, I get a very small print shift along the Y axis. This shift is literally less than 1mm, I suspect it to be close to 0.65mm. The shifting is pointing towards the inside of the printer, which might help, I don't know. According to the build guide, it could be due to a number of problems, including too loose or too tight belts. I don't consider my belts too tight, as they don't require any force to push down at the center; nor do I consider them too loose, as it doesn't loose grip on the pulley on the stepper motor.

I'm lost, but it makes it really difficult to print interlocking parts that need to be precise when I've got this very small shifting.

Here's an image to show what I'm talking about, even though my camera wouldn't focus. It uses the aluminum extrusion as a reference point.
Attachment 2524