You are correct Western only recommends 100 grainsv BH as Hodgdon does for loose T7 but each ML company does their own independent testing and then post recommended loads for the different powders in the the manuals for each rifle.

Companies producing the powder always have to consider ALL ML rifles and the abilities of ALL ML's to handle a load of powder. Because of that Power companies always post very conservative loads to try to cover some of liability of a person shooting these loads in an older or weaker ML.

Although I think Western has loads posted on their web site for loads greater that 100. I do not shoot a lot of BH so I can not say that I have actually looked on their web site for recommended loads.