Yes, that's where the "engineering" part comes in.

Engineering builds models that approximate the real world under practical considerations. You e.g. have to weigh costs vs risks and apply good judgement.

In essence, you try to achieve the best possible with the resources you've got.

You are describing the difference between Verification (Does this do what we explicitly said it would do?) and Validation (Does this do what we wanted to achieve?). One is about compliance of design promises, and the other covers the issue that the various levels of design documents might have failed to completely capture intent.