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Here’s another way of thinking of it:
You want the oil to be at the highest level possible within the correct range(in other words, at the upper dot on the dipstick), during normal running conditions. We do this to ensure proper lubrication while cornering and braking and avoid submerging the crankshaft with too high an oil level. The catch is you can’t check oil while the engine is running because taking off the dipstick often causes a vacuum leak big enough to trigger a check engine light, as well as it is less safe to the average consumer(this is why many cars switched to sensors instead of dip sticks). So that’s why instructions say to drive your car until oil temp stabilizes, turn off the engine, and immediately check oil.
-Checking while cold will give a false lower reading because it takes longer for thick, cold oil to drain back to the sump.
-Checking long after shutdown will give a false higher reading because all the oil will drain down from places like the head and oil filter that would normally have more oil when running.
-And sometimes the opposite is true because volumes change so much with temperature and all the more reason everything needs to be at normal, running temperatures.
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