The eye is not very good at judging distance. If you miss the targeted length of a bookshelf with a millimeter or two, it will be very hard to see. But the eye is very good at comparing and spotting differences. Thus your woodworking results will improve if pieces that should fit share the same size.
If two long separate pieces differ in length with 2 millimeters, the eye will likely not notice it. If you place the two pieces with one end flush, and look at the other end, the eye will immediately see the difference of two millimeter.
If this would be parts of a bookshelf, where a gap exist between the bookshelf side and the shelf, the eye sees it easily. But if all shelves are shorter, it becomes a repetitive pattern and is not noticed by the eye. If all snug up perfectly except one or two, those will be easily noticed. Your woodworking improve if repetitive cuts repetitively give the same length.
Making perfect measurements might be important, but more important is that the length or width are repetitively made the same. With improved repetition, differences becomes small. It might be slightly wrong length, but as long as the length is the same for all pieces fitting into the same pattern, everything looks fine.
Thus, improve woodworking using repetition. Do your setup so that all pieces needing a certain length are cut to the same length. Target the individual differences more than the absolute measurements.