Is it normal comment number discrepancy

I don't know whether it's a bug or a feature but there are so many posts with missing comments. The advertised number on searches does not reflect reality. I used to think it's when OP hides comments but it even happens on my own posts and I didn't hide any.
It could also be the site owner censoring stuff he dislikes, or a bug with the way SQL tables relate to each other.
Something annoying as a result of this are bumped 7 years old posts where you can't find the recent comment that bumped it.