It's not like this is a reasonable request. Part of the issue with something with a very low probability of happening is you can't really just go and make it happen on a whim. Especially in this context. Not like everyone is just screenshotting or frapsing their mission windows constantly waiting for this super low probability thing to occur. When it does, evidence of it is gone in seconds. There's no mission history or log to look through.
I've never seen it happen personally, but what he is describing is perfectly legitimate as something that could happen. Rounding is a thing that happens, and failing something with a 99.5-99.99% chance of success is a thing that happens (especially in a game with several million people). Just because you haven't seen it and the chance of it happening is very low doesn't mean it can't happen. That said I've never seen any official confirmation that this is what's going on. And it could also just be a display bug somewhere that's unrelated to rounding. Some follower counter or other contribution being calculated and displayed as more than what the server counts it as. It seems unlikely at this point with how many times the complaint has come up that literally all of them are lying (especially with how little there is to gain, since Blizzard almost certainly will not do anything about it on an individual basis).