That's not true unless they either changed something for Mischief or just not enough people there. It has a variance in base pick the same as all mobs do in a base pick which is calculated from the total number of players in base + all picks. Picks generally don't get a variance or a very little one. On Mangler the spawn time of ground spawns in SONH and Kaesora in base was anywhere between 50 minutes to 65 minutes. It was 50 minutes up to 80 minutes on Aradune. It can spawn as early as 50 minutes if you are the only person in the zone and up to 80 minutes or more (1 hr 20 was just the highest we saw on aradune) if there is an absolute ton of people there.
It's the same thing with raid mobs except raid mobs have an additional variance that was added during classic on Mangler. So raid mobs get zone load variance on top of having a raid mob variance. The combined % of both variances in terms of raid mobs starts at -25% (meaning a raid mob can spawn up to 25% faster if no one ever enters the zone) and starts to go to the positive as more and more people sit in the zone up to as much as +200% or maybe even more (meaning a raid mob respawning in a heavily populated zone can take up to 2x+ longer to spawn).
For example vox can respawn as early as 17-18 hours if no on ever exp's there. Depending on the population of Mischief you should probably be currently seeing Trakanon as a great example. At the beginning of the expansion he takes forever to respawn and it should get faster and faster as the expansion goes on. By the end of the a normal 3 month kunark expansion Trakanon will respawn every 42 hours or less consistently as Seb becomes less and less populated. On Mangler the longest was 3 and half days which was the time between the server first kill and the next respawn and steadily decreased in respawn time from then down to its normal respawn. On Aradune we saw as high as 4 and half days multiple times, but by the end of the expansion it was down to 42 hours or less consistently.
This is why in TEB we always asked everyone afk waiting for raid mobs to spawn to never afk in the zone that the raid mob spawns in. For example in Kunark, we didn't let people AFK in VP/Sebilis/Karnors, they had to sit outside. Alternatively you can also use this to your advantage. There were a couple instances where we actually asked everyone to come AFK in a specific raid zone because the respawn windows were closing during a bad time in the AM hours, so we would overload the zone with players for a while to push the respawn more likely in to the PM hours.
Interestingly though, it was noticed on both Mangler and Aradune that for whatever reason Kael specifically is not effected by these added variance timers. Maybe it's coded as a city therefore doesn't apply? Not really sure.
Edit: Forgot to mention that the variance also does not apply to Vulak trigger. Vulak trigger respawns every 84 hours (if I'm remembering the respawn time correctly) no matter what.