The ever changing torque specifications of bolts

Depending on when you look, the torque specs for any given bolt have changed, not insignificantly, over the years. For those in the know, is this because of some change to the bolts themselves, or is this solely due to changing knowledge/testing/data? 

I thought it was always two uggas of torque, or guten-tight specs for the german hardware. Seriously interesting question though.

You have to be more specific, do you mean climbing bolts or just bolts in general?

I can think of two such examples. The first was the case of the published installation torque specs for the Rawl/Powers "Power-Bolt" aka: "5-piece", in 3/8" carbon steel. Some bolts snapped upon installation with a torque wrench at the manufacturer's 35 ft-lb torque spec. It required a few discussions with their engineering department to persuade them to revise their published torque value downward to 20 lb-ft. Any automotive mechanic should have recognized that 35 ft-lb is far too high for the 5/16" diameter, grade 5 carbon steel, inner bolt. I can only guess that someone in the company didn't realize that the 3/8" nominal anchor diameter had a skinny internal bolt and chose a torque value more appropriate to a 3/8" grade 5 machine bolt.. Here's one of my old posts:   

http://www.supertopo.com/climbing/thread.php?topic_id=2927238&msg=2928033#msg2928033 

The other example is that the published torque specs were reduced by another manufacturer for their stainless steel anchors. Torque values for stainless steel are considerably less than for hardened (grade 5) carbon steel.  It was the Powers 3/8" SS "Power-Bolt" which had torque values reduced. http://www.supertopo.com/climbing/thread.php?topic_id=277206&msg=280136#msg280136 

Dewalt / Powers actually increased the torque spec recently for the SS 3/8 and 1/2 bolts. A small amount...

I've seen SS 3/8 5-piece as low as 10 ft-lbs and now 18 ft-lbs. That's an 80% increase in torque!

It has been 28 ft lbs for SS Powers 3/8" wedge bolts for a long time. At least  20 years.

I left out that i was refering to SS "5 piece" bolts.