Hodgdon purchased IMR in 2003. For whatever reason Hodgdon continues to use the IMR brand name on certain powders, including the new Enduron line.

The annual Hodgdon manual from 2010 has an article written by Layne Simpson about Trail Boss. He writes about spending time in July 2003 with J.B. Hodgdon at Thales Australia Ordnance Land Systems, the manufacturer of Trail Boss and several other powders Hodgdon distributes. He writes about going to their range and shooting a new powder on which final development had just been finished. That powder would become Trail Boss. Obviously any official data printed about Trail Boss comes through Hodgdon, regardless of whatever brand they choose to print the data under. I know, just so 2003, right?

Interestingly, Simpson relates a conversation he had with Ron Rieber, who had been Hodgdon's head ballistician for many years. He quotes Mr. Rieber as stating that it is impossible to get enough Trail Boss in a case to exceed maximum chamber pressure. Rieber was also quoted in the June 2016 issue of Handloader as saying Trail Boss becomes erratic, not dangerous, if compressed. I believe that Mr. Rieber has probably forgotten more about ballistics than I'll ever know, so I choose to believe him.

Also interestingly, my .357 brass with a 158gr. Speer swaged SWC will hold 4.2 grains of Trail Boss to the bass of the bullet when the bullet is seated to the crimp groove. 70% of that is 3.0 grains. It would appear that Hodgdon printed their data with prevention of compression of the powder in mind and chose to back off just a bit because of the differences in molds and alloys involved in casting. The Speer bullet is pure lead and will be shorter than most cast 158gr SWC bullets.

I have no idea where the O.P. got his data from. He should repeat and reweigh the charges he got if he actually followed the the "fill to the base and call that 100%" rule. If he didn't get his charges by actually weighing powder, he should do so if he wishes to use Trail Boss, or simply use Hodgdon's data.