Sometimes a fight bugs out, or is just out-dated, and thus some scores are literally impossible to beat.
Say X boss get's a health increase and enrage time increase on Lordaeron, that affects the max possible kill speed.
This also holds true with rebalancing, mechanic changes, etc This means the leaderboards showcasing things from old OLD content isn't the same as up-to-date content.
I suggest having the leaderboards fixed to better suite more up-to-date showcasings of skill, thats all :)
could be fixed by adding some form of note to the old score, either generic "outdated score" due to a boss fix, or "unreliable score" due to a player ability fix
or, what everyone wants...
"this boss' hp was increased by 10% and its damage output increased by 5% on melee and 15% on spells"
put that in a tooltip, and in, e.g. 2 months from that patch, bump the timer down by the ~10% average difficulty increase