It's because there was a significant jump in a short time. How else can you explain it.
So what you're basically saying here is that if a handful of people are exposed to a piece of content at roughly the same time, review it at roughly the same time, and share their (independent) opinions on it at roughly the same time, and if those opinions happen to align against - then those people are rigging votes rather than accurately representing a different perspective on said piece of content?
You can depart solo if you want practice... but, like another poster mentioned, these are long multi-phase fights so chances are you won't be able to see the later phases anyways. A solo friendly practice mode with severely nerfed health is something I could see them adding but not something I'd personally want to see in the game as it would diminish the impact of the content for players who are going into it for the first time.
Don't overthink it, just join raids, die, and do your best to apply whatever knowledge you've gained from videos and previous runs. Yeah it's unforgiving but also more satisfying when you feel yourself getting better with every run. We've all been there - there's no shame in it.