Research has shown that if you X it by 1.8 you have a number representative of the total infestation when brooded up, rember the bulk of the mites are in the capped brood see Lee Et Al (2010),
https://articles.extension.org/medi...s_for_Varroa_destructor_in_Apis_mellifera.pdf
Most just 2x it for simplicity
You have to rember a wash gives you the percentage of mites to bees in the brood nest. But its not telling you the advrage mites poric mites in the whole hive (with out more math) the brood nest is a much higher number, and its used as its the most stable and repeatable.
as such its just a number, that the bean counters then correlated with the effect on the hive, then backed it up to come up with thresholds that are predictive of pending economic impacts on the hive.
look at it this way… shooting spitball numbers- A bee lives for about 6 weeks and is a nurse for about 1 week, so most of the phoric mites are on 1/6 of the bees
so if you roll 6% on nurse bees that really means something like 2% phoric and 10.8%(6x1.8) overall infestation(if you have brood). So if you roll 6% with no brood its about 1/2 the over all infestation then if there was brood.