Well, the round may be at an angle during feeding. I notice that once the case bottom clears the mag lips, the mag spring tension on the follower pops the case up the slide breech and lines the case straight with the chamber and the slide continues to move forward ending with the case neck seated against the chamber with the slide locked against the back of the case.
It is VERY CLOSE to straight. But it is not straight. breech faces are cut at an angle. That's why if you want to convert a .40 pistol to 9mm you can get a conversion barrel (smaller bore diameter lets you account for breech face angle difference while leaving sufficient barrel wall for the pressure), but not the other way around.
Also, the barrel is moving and changing relation to the slide/breechface/extractor right up until it is fully in battery. Your mag spring just gets it under the extractor hook. The act of chambering finishes lining it up. Otherwise it wouldn't touch the side of the chamber while chambering until it was almost all the way in. I haven't run into any semi auto that doesn't have some interaction between the loading round and barrel hood as things go up the ramp.
Like I said, I have one gun with an AET barrel where they make a point of trying to ensure it chambers concentrically. It exhibits the stain in multiple jets where the chamber fluting is rather than on one side.
So why doesn't similar burn rate powders like Bullseye and Red Dot do the same thing? My Bullseye loads do not end up with burn marks.
Some other powders DO do the same thing. Just not all, and titegroup is by far the darkest, and usually makes it the farthest down the brass. also, other powders tend to only do it noticeably when loading light. For example, 110pf .40 loads using zip do it a bit in my M&P, 137pf loads don't really do it much at all, and 155pf loads don't do it at all. Titegroup will do it at max book load and beyond. I suspect it has something to do with it being a double base (although I think UC and zip are both double base), or with the flash suppressant in it.
giving it even more thought, UC is really bulky there's more per pound, and more per FPS out the barrel. Zip weighs less per pound than TG, but you use more for the same FPS, so in actual use it is a bit mroe bulky than TG. Maybe the fact that you never really fill a case with teh stuff and it is lying on it's side makes a difference. TG is supposedly deliberately engineered to be insensitive to powder distributon/placement. I always assumed that meant it kind of flashed over.
In the end, the TG stain is pretty irrelevant. It's a pretty accurate powder with decent standard deviation. My problem with it was when shooting rapidly, it would get my slide unpleasantly hot, and couldn't be doing good things for my barrel. It was also causing issues with plated and moly coated lead when trying to make major power factor in .40.