This is something I’ve seen several times. often in aerospace the threads are supposed to be lubricated but if you get lubricant under the head you can get way

This is something I’ve seen several times. often in aerospace the threads are supposed to be lubricated but if you get lubricant under the head you can get way more preload than you expect. Same for changing the washer material for a thermal or other reason and not realizing it has much lower friction. For unique applications in aerospace, it’s common to do testing to determine a non-standard torque-preload curve. In some industries, preload is measured more directly using hollow bolts (so you can measure the before and after length therefore determine preload), ultrasonic measuring devices (same reason) or preload sensing/indicating washers (there are many types).