From what I can gather there are two competing theories for how boss mobs work.
The first theory:
All boss mobs have a minimum level. They will scale up to match player level+3 but not scale down past point. In this world karazhan bosses would work as lvl 73 mobs of for a level 70, lvl 74 for a level 71 but would be lvl 73 for anyone between levels 1-69. This type scales only up.
The second theory:
There are two kinds of boss mobs. The standard Raid boss who is always player level +3 even with a level 1 character. This first type of boss scales both up and down. There are also special case mobs who at a set, seceret level. In the case of training dummies and Racial bosses would appear to be lvl 83.
Either of these cases can be the truth. easist way to test which is correct is to get a lvl 69 tank into kara and watch a parse to see if they take crushes.
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Majiben