The game takes RNG into account to predict results because there's so much going on in any given round of combat that no player could be reasonably expected to calculate it. Like, you know how Fire Emblem Heroes is basically totally unplayable without a matchup calculator now because every weapon has a full kit's worth of effects, on top of all the other effects in every character's kit? It's basically the same thing, except with all the pay to win, obtuse nonsense replaced with actual teambuilding depth.
Personally, I think it's a good thing. Random unfavorable aberrant chance results (high hit misses, low miss hits, low chance crits) are the most hated thing in this genre, and a system that essentially eliminates these as an issue is really good game design. Fire Emblem should take notes. Fire Emblem wishes it was this good.