A hechsher is only as good as the rav hamachshirs ability to manage those who work for him, often hundreds, possibly thousands of miles away, across the ocean on another continent.
Often hechsher “a” would never hire certain people as mashgichim, but those people end up the mashgichim w/o anyone knowing because what happens 1000 miles away for 45 minutes can be kept quiet. “Sticker rav” may not hire me, ever, yet i might be “covering” for his guy who slept late, had to go early, called out sick, etc…
To answer mdlevines questions. Who says the sticker does anything BUT rely on the other hechsher? Remember the vynegard fiasco a few years ago, just before pesach? Every “sticker” relied on one kashrus agency, affixed their names and when the agency announced an issue all the stickered products had to be recalled to.
Again, more often than not the hechsher is a marketing tool. Which one, where to put the hechsher on the package, if at all (in the case of national brands).
I once called a major kashrus agency when i saw their symbol on a containder of milk. I asked them if their symbol meant anything as i was not makpid on chalav yisroel anyway. Did the hechsher certify something more. The answer was no. They rely on R’ Moshes psak regarding chalav stam and regarding the symbol on the milk carton “thats because we have a contract to place our symbol on all kosher items”. I guess, even when not necessary or when it is meaningless.