Same thing happens with Virtual Horizon features on cameras.
Welcome to Manufacturing Tolerances 101. In my experience with hundreds of DSLRs, I've found none to have 100% alignment. In one case, I found an expensive new camera to have a viewfinder that was more than 2% off. In no case have I found perfect alignment between camera bottom, hot shoe, sensor, and viewfinder, and it's common that I find I need a half degree to one degree angle correction in post processing using almost any of the leveling capabilities supported by my equipment. The only reliable way to get absolute level images is to use Live View, and then only if you have something visual you can align to the gridlines.
For some reason, Nikon seems to be perfectly happy with "downhill right." Every Nikon DSLR I've used (which is all of them, and many in multiple copies) is downhill right when aligned on a tripod that's absolutely horizontal, or aligned by viewfinder to the horizon. Every Nikon DSLR. The fact that I've never seen a Nikon whose alignment is downhill left says something important. That means that Nikon's jig in manufacturing for positioning the sensor is structured that way: half degree off, downhill right. The viewfinder may be "more off" of "less off," which explains the small variation that people have encountered.
This gets me back to one of my pet peeves about Nikon: if you're not close enough to hear your customers, you just keep making the same mistakes. No one's ever complained enough that it got back to manufacturing to look at the alignment, let alone the tolerance.