In summary:
Threaded fasteners can clamp materials together only when they are holding with the proper amount of tension. For this to happen they must be properly tightened. To this day a simple, inexpensive, and effective way to consistently and accurately tighten a fastener does not exist. There are a number of tensioning methods that that do this better than a simple torque wrench but they are both complicated and expensive. In most situations, the less-than-perfect traditional method of a torque wrench is sufficient.
Why is a torque wrench with widely-varying known errors sufficient? Because engineers compensate for the inability to consistently and accurately determine bolt tension by massively over-designing joints. This accommodates inaccurate tightening and avoids catastrophic failure. Designers will specify more or larger bolts than needed in order to ensure that the joints are sufficiently clamped together. It is true that fewer or smaller fasteners could be used if bolt preload was more accurately and consistently controlled - but that is not practical nor realistic. And historically, the over-design of the fastener has been far cheaper than controlling the tightening process in a factory (and which is nearly uncontrolled in a DIY/backyard mechanic scenario).
Thus, the fairly large inaccuracies of measuring tension using a torque wrench are taken into account during the design phase such that large errors in applied torque will not result in failures.
What does this mean to you? Simply use the best tool that you have and torque to the specified value.
And now you know why manufacturers use 12 bolts to hold two parts together when it sure seems like 4-5 could have done the job!
