My group never really experiences a problem with this. We're very clear that the characters in the game are built to reflect how a narrative unfolds, not to reflect a simulation of their actual abilities. So the reason The Flash and Sloth Man get to both have the same mechanical number of actions, regardless of description, is because the story is about how the two work together as a team, not about how The Flash takes out 6 thugs while Sloth Man only manages to take down 1.
It's similar to how, when a Conflict starts, you break it up into groups like "The Flash vs 3 vampires" and "Sloth Man vs the evil wizard", and even if The Flash beats those 3 vampires before Sloth Man has beaten the evil wizard, he can't interfere, because his part of the story for that Conflict was about him fighting the vampires. Fighting the evil wizard, at that point, is Sloth Man's part of the story.