So this is just going from memory.
Two decades passes between ROTS and ANH. ANH Vader you say can't beat ROTS Kenobi. ESB is what, 3-4 years after ANH? THAT Vader has a 50/50 chance? And then ROTJ, 1 year after ESB, Vader for sure beats ROTS Kenobi? He improves more in like 2 years than he does in 20? It takes him that long to get back to his old self? Seems weird that each Vader in each movie has a clear power level. I rather think all 3 incarnations are closer to each other than most claim.