In normal conditions it’s only like a 10% or less gap from worst to best.
The big differential seen, as somebody has pointed out earlier, in the very top parses usually stems from how fast a kill was bursted down, or the class being designated in a fight to deal with something it excels at, and example being designating specific high on demand burst specs on shades while the rest of the group nukes barghast to prevent an additional set of shades.