The idea for me is that you blow or don’t blow based on clear and obvious difference to what is expected (or likely to have happened).
To explain with a couple of scenarios:
Forward pass – People expect passes to be backwards, so it has to be clear and obvious forward to blow.
Dropped Ball – from a kick if the full back drops it, people expect the knock on. So it has to be clear and obvious that it was NOT a knock on to prevent the whistle.
The whole point of clear and obvious is so that people understand and believe the decision. Not to prevent the whistle being blown (but it helps with this aspect as well in most scenarios)