so, I torque it to 6Nm then I keep turning it another 21* regardless of torque?!

Torque is a rough way to figure out how much clamping load is on a bolt, and generally is not particularly accurate. Imagine a nylock nut for instance. If you have a shiny new M16 bolt and you've lubricated it up well and you tighten it to about 50 inch pounds, you can expect that fastener to be putting about 500 pounds of clamping force to hold your two metal bits together. If you however take a shiny new M16 bolt and use instead a nylock nut and put 50 in-lb of force on it, you've create approximately 0 pounds of clamping force because you need more than 50 in-lbs (approximately 70-ish iirc) to even turn a new nylock nut on an M16 bolt. Torque in this case is completely different for the two fasteners. In this case it was because it was a nylock nut, but it could be because it's rusty, because a different type of lubrication was used, because no lubrication was used, because one bolt was yellow zinc and the other was 304 stainless. There's all sorts of things that can change our clamping force when we use torque as a measurement to determine clamping force.

Now lets say we've got our two M16 bolts again, and we've got a nice shiny regular nut and a nylock nut. Now we're going to drive both nuts down until there's no space in between the bolt head/washer/nut/gasket, or whatever it is we're wanting to clamp. We drive it down once to seat everything, we back it off so we're at 0 clamping force again, then we set it to a known but very low clamping pressure. Let's say 10 ft-lbs. Then we're reasonably confident we're putting enough pressure in the whole system that there's no empty space in there, but it's not enough pressure to stretch anything in any significant way yet.

Now what we want to do is rotate it say 10 degrees. There's about 12 threads per inch on a coarse thread M16 bolt which means one rotation is 0.083". We've done 1/12th of a rotation, meaning we've stretched the bolt now about 2 thou from what it was before. As long as we know what the bolt material (and in some cases gasket and clamping material) is and the length of the bolt between the ends we can calculate what our clamping load is going to be because we know exactly how long we've stretched the bolt. Doesn't matter if the threads are dirty or lubricated differently from one to the next.

An even better way to ensure you've got the proper clamp load is to just measure the the stretch in the bolt itself. This is often done with connecting rod bolts and a gauge, but it can be difficult in blind hole situations.