There are no hard and fast rules about what precision a router can be built to hold. But tolerances for things made from wood aren't going to be as high as those for metal objects. Wood is inherently more difficult to machine to a precise dimension, if only because it will swell and shrink with the weather, changing dimension markedly as it does so. Metal parts, on the other hand, while they will expand and contract with temperature, do so in a more controlled and uniform way. They often need to fit into one another rather precisely - a sliding fit, for instance, is different from a press-fit. So you'll rarely find a milling machine that can't hold a couple of thousandths of an inch one way or another, and some of them can hold "tenths" (of thousandths) or even better.
It's certainly possible to build a router to that degree of precision, but every arithmetical increase in precision entails a geometric increase in cost, so one has to ask "why bother?" if the result can't be measured and doesn't matter anyway.
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