AimIsADickCalling damage ≠ Calling health. e.g Calling "Demo took 50" means that demo can be from 50 to 210 due to overheal, while calling "Demo IS 50" simply means that demo is at 50. You'd be suprised how many times people lost simply because they guessed the wrong amount of health, since they tracked using damage numbers. That is why Solemn Vow is so useful.
how can you be so wrong about how the game actually plays with such a high level of confidence in yourself
solemn vow is dwarfed in usefulness by proper focus calls, if your team is playing together properly, you can call focus on a player with any amount of HP and kill him if your positioning favours the play, you can reasonably call focus based off seeing shit that's only about to hit its target, stopping to visually confirm a player's HP before making a call rather than just communicating what people are doing to who and calling proper focus is wasting time that he will use to reposition, and every moment wasted means your chance to make that play evaporates