It’s a function of the diameter, lubrication, and strength of the bolt material.
Once the metal plies are in contact any further tightening will put tension into the bolt (this is good). This tension makes it harder to further tighten the bolt, so you need to apply more torque to tighten it. The specified torque is to stretch the bolt enough that it loads up with enough tension to reach a defined value relative to the bolt material strength. This exact value varies based on the bolt material properties.
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