Buzzbox, we are also encouraged to report "just in case" which is why the majority are inspected, investigated then found to be below the threshold.
The unintended consequence of this, particularly for someone who has previously reported a suspected heavy landing which was subsequently found to be below the threshold, is that your expectation shifts. By that I mean you land the aeroplane with a crunch, go "oh crap I had better write that up", they inspect the airframe, look at the FOQA data and the feedback is that the landing was well below the heavy landing threshold. Next time you go crunch you go "yeah that was a thumper but no worse than what I did last time and that one wasn't even close so no need to report it" or words to that effect
Nothing sinister but it leaves room for the occasional one that goes the other way and does exceed the limit but isn't reported. Sure some might try and cover it up but in the operation I work for I would think (hope) they are in the minority, with the unreported ones there because the crews genuinely didn't think it was that bad.