As well as the comments already made, there's another thing to consider: in a SLR or DSLR, the path used to determine whether or not you're in focus is different from the path light follows to the medium: the autofocus system has a screen that takes light from a mirror, and then adjusts the lens based off that measurement. In manual focus, you're basic it off the path the light takes to the focusing screen, via a mirror.

In either case, when the photo is taken, the mirror whips out of the road (which generates mirror slap, another problem), and the light travels to the film or sensor. Since this path is a different distance to the one used for focusing, it requires only a small calibration error between the body and lens to end up with out-of-focus images. It is very normal to have to tweak the focus (higher end bodies will let you do this), or to have to have a body or lens physically corrected. No matter how exquisite your optics - in fact paradoxically, this effect is worst with high-end optics - the way the light path works here can be a real pain for getting good focus.

Because mirrorless systems focus and capture from the same medium, this literally can't happen with your mirrorless camera.