Your point is only valid if you want to argue that the raid is going to execute at the same level and take the same amount of damage every attempt. I can assure you, if you actually do Mythic progression, it's not uncommon to see 20% variances in the amount of raid damage taken across attempts of the same length. Maybe someone failed at a raid damaging mechanic (let's say they blew up a bomb next to the raid on Anomaly, or blew up a brand when the strat was not to blow them up on Tich). Maybe people are better at avoiding avoidable damage one attempt vs the next.
Let's say the raid will take an average of 10 billion damage on a kill, but the actual range can be 8 billion to 12 billion. You ideally want to plan your mana around having enough longevity/throughput to heal at the 12 billion damage taken level, not the 10 billion average or god forbid the 8 billion best case. If you waste extra mana and generate unnecessary overheal because you want to maximize your HPS percentile/ranking around the 8 billion best case, you are likely to be OOM the last 25%+ of the fight, which is likely to cause a wipe, which is at least partially on YOU being more concerned with padding than actually successfully downing the fight. If you play conservatively to have the ability to respond to that 12 billion edge case or close to it, you're going to end up with some excess mana at the end of the fight, and that absolutely is not poor play - it's being intelligent and understanding the healing role as opposed to being an immature healer that thinks HPS = DPS. You aren't necessarily going to be able to predict who is going to fail a mechanic and "bomb" the raid 3 minutes from now; you need to predict an acceptable range of action, and have the needed leverage to expand longevity and throughput to respond to unpredictable events - at least if the alternative is wasting mana to pad on non critical healing.
Like I said, go balls to the wall on deep farm fights (which I would hope any heroic fight would be by this point) if you are obsessed with ranking numbers, but it's not something that is relevant or all that indicative of healer skill on stuff that isn't trivialized.