xkcd #3138: Dimensional Lumber Tape Measure
Title text:
A person with two watches is never sure what time it is, especially if I got them one of the watches.
Transcript:
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Source: https://xkcd.com/3138/
Modern lumber is planed, so some of that difference is because of losses from that. If you open up the walls of a house built 100+ years ago, you see these thick rough wall studs that never went through a planer. Even with shrinking, it's close to being actual 2" x 4".