Ya, drill/space is more a matter of "nah, she'll be roit!" versus inspecting things with a pocket loupe and fine-toothed comb. Normally when you order 10, they make 11-13 and discard the bad ones; their losses go up for finer tolerances, so they have to charge more for more waste. But it's almost constant labor, until the limitations of their process (printer raster pixel size?) become dominant.
Also, fine drills break easily, or really fine holes have to be laser drilled (pricier equipment? slower?).
Four layer boards take more steps obviously, and have to be lined up precisely (by x-ray, no less), however they're bonded. (You can make two 'core' boards and laminate them together with one thin 'core' between them, or 'build up' each side, having relatively close top and bottom pairs. The latter is more common, as it gives reasonable width ~100 ohm traces for high speed work. The former would be good for extra supply bypass (closer planes = more capacitance, lower impedance supply), or internal routing with supply pours on the outer layers.)
Tim