Keep in mind when manufacturing that everything has a tolerance, because no manufacturer can make any thing perfect. It just can't be done in our world and if you really believe it looks and feels perfect, it' not. Engineers always use an allowable tolerance when designing components. The diameter of your battery may be ±0.3mm. The machined cavity depth (between the door and the back wall) may be ±0.2mm. So the tolerance of both components together is +0.5mm maximum and -0.5mm minimum. That's a difference from max. to min. of 1.0mm (approx .039 inch). Theoretically, if both components are max., then it's a tight fit. But if they are min., then it's a loose fit and the battery may slightly rattle with 1.0mm of dead space.
The tighter the tolerance, the more it costs to manufacture, because inspection time goes up. Let's say a part coming off an automatic machine would normally have inspection of certain characteristics every 10th part, but with tight tolerance work, inspection of every 5th part is required. So, inspection time doubled and that time is incorporated in the cost of the part. Fall-out (scrap parts) is inherent to manufacturing and the tighter the tolerance the more scrap there is, which means more production time and material are lost.
Why does the inspection time increase with tighter tolerances? Because there are many factors for an item to easily go out of tolerance, like tool wear, improper handling, temperature change and other factors. Some factors are predictable, but many are not.
Manufacturing is usually a trade-off between what you want (perfect) and what you can economically produce (practical). It's not uncommon in some industries where inspection time is the controlling element of an item's cost, where it cost more to inspect the item than it does for material and production time.
