In real world engineering, defects are part of the design and not bugs. Really they aren't even called defects, because they are inherent in the design.
Maybe you bump your car because you stopped an inch too far. Perhaps it's because the tires on your car were from a lower performing but still in spec batch. Those tires weren't defective or bugged, but instead the product of a system with statistical outputs (manufacturing variation) rather than software-like deterministic ones (binary yes/no output).
Which goes back to OP's initial point: SWE types aren't used to working in fully statistical output environments.
