It's happened to me many times.
I'll do awfully on a stage and get an S on the victory screen.
I'll pull a CrushPunch performance and get an A or B.
Even maintaining berserker mode without dying has its own level of difficulty!
I posted this before, but I cleared stage 6 with an S at the victory screen during a Hard run in a normal game, after losing my last ship (down to 0 lives) and taking almost 3 minutes to kill Grubenriese.
Then I attempted to fly the Rating Master challenge, and in the same stage, pulled a stellar performance, no deaths, very few enemies missed, beat Grubenriese much faster.
Victory screen? A. Failed.
My friend did make a good point, though: when Delci is around, strange things tend to happen. There's like a certain radius of fluctuating luck surrounding me at all times, haha.