any human being is fallible unless we have a proof
historians were more or less story tellers
that might be a hard qualification
as they also relay on what they heard or witnessed
so there's what we may call science in their work
So we should use our mind and make a qualification if possible
as we know Quran is against blindly following
And in the equal sunni narration if I'm not that wrong the prophet used this expression: of let or may the present tell the absent
but honestly I'm not looking for texts that emphasize on the superiority of one sect
or on a clash between sects
I'm happy when Muslims find consensus
The point is we don't really know the knowledge that was accessible at that time
so if in a hadith they say 500000 maybe they didn't know numbers like a million or billion
and if they knew they would use it
as that would give more emphasize
on the "magnitude" or "dimension" of a good or evil act etc.
like when Arabs say 1000 Marhaba = 1000 WElcome
it only shows hey you are very welcome
So maybe there's a shi'a explanation for your Question
Maybe there's even a different sunni explantion as the one that came into my mind
I hope you'll get an answer
With my comments I just wanted to point at some possible Issues
As we don't have an attendance list for that
Maybe 100000 were present and somehow heard his words
maybe some didn't hear it directly but via the intermediate of those who were close to Muhammad
@Meysam I hope I could make my point clear