I'm willing to have this discussion as long as it is productive and not finger pointy, which the discussion has always devolved to when I've seriously tried to have this discussion.
The dirty little secret about SP vs SP in MFN is that it's not actually about the SP of the player, it's about the player's physical weight.
The reason that WRs do better at RB, even accounting for equal SP, is that WRs will inherently accelerate faster than the RB because they are 20 pounds lighter. The AC score is not normalized by weight -- the max AC has just been adjusted to show the actual max value a player can achieve.
An 80 AC DE will always accelerate slower than an 80 AC WLB -- hence the McKeon effect -- and an 80 AC WLB will always accelerate slower than an 80 AC CB. All the AC means here is how fast per position the player will get their weight moving compared to other players that weigh the same weight. (Confusing right?)