I expect that is really just fancy wording. While I have not tested it, I expect all the test dummies have the same base resistance with the exception of the iron atronach (sort of). Resistance is very consistent in group content. iirc ~18k and I am sure someone can correct me with the specific number. The exception if vMA where the resistance varies wildly.
The reason I say the Iron Atronach is an exception becasue it provides raid buffs with include major breach and fracture since in a well-organized raid the tank would be applying those debuffs. It is the DPS dummy that most mirrors what one would expect in a decent raid group for buffs and debuffs and provides shards for resources. It is good to use since one does not have to adjust their rotation to take into account the lack of buffs.