All good points and I would offer the following;

1) It matters whether the bolt threads are lubricated or dry when torqueing.

2) It also matters whether the bolt threads are brand new or worn in a tad.

3) Even with name brand bolts it is possible to get a bad one on occasion.

Not all bolt applications require clamping to 90% proof load so sometimes it is hard to find a bad one. If you are clamping to 90% proof load than 1 and 2 above are critical.

Kitz