I think the baseline problem is assuming the Munitorium guy has either Int 30 and some training, or Int 40 and no training. Also, situational modifiers (the munitorium guy actually does have a "stronger position" with regards to being able to control this).
If you assume 40 int (human baseline 30 + 2x +5 Int advances, being that its his primary characteristic), +20 to trade (once again, primary skill), and proper "equipment" (cybernetics, dataslates, cogitators), you're looking at a 70 in the skill. Can easily come up with reasons for a +30 situational modifier.
this modifiers are way too high. a normal IG doesn't even have Dodge + 10 (I think) and you would think, that you train in a skill that your life depends on. Also with these kinda stats you don't need to try trade commerce at all, if you are not maxed out the wazoo. And if you adjust your normal npcs(not boss enemies) to the powerlevel of your PC's, why give them exp at all? After all. EVERY Munitorum guy in the IG would have to have 40 int +20 Trade and Cybernetics.
Also, if you think about, why is it an opposed test? Either you know how to work the system or you don't. For it to be an opposed test, the munitorum guy would have to actively try to deny you your request with his knowledge of the system, that's insane. Couldn't he care less if you get a plasma weapon (and do your job better because of that)? The system was put in place by the infinite wisdom of the emperor and his staff. So if the system says: ok, who is he to try and work against it? HERESY!
Maybe this test should'nt be opposed, and every 2 SL should get you a +10.
Actually Munitorum officials are more akin to office workers than you front line Imperial guardsmen. This means they are mono-task workers and so are highly likely to be more skilled at their job than a more generalist tool of the Imperium such as a Guardsmen is.
As such YES they will actively try and deny you making their life a nightmare, with loads of paper work to fill out, sign and send off in triplicate, communications to forwards up and down the chain of command and heaven forbid they have to actually travel and talk to someone to do your leg work for you! They are glorified calculators and logbooks and do not care why anyone needs to acquisition anything. If you need something and you have clearance for it then as long as you go through the proper channels they will comply, no questions asked. But if you are trying to game the system in your favour, or are outright trying to con the Munitorum out of gear, even if you really, REALLY need it, then the Munitorum officials will of course oppose you.
Your position in the imperial guard is as much an insignificant statistic to the Imperium at large as you are a bit of punctuation in any one of the millions of reports that official will have to fill out in his boring, monotonous life. Be happy they are willing to supply anything beyond standard issue gear to you at all i say!