Thanks Mschmidt. Yes, differences of culture density could easily be one factor. For example, think of some of these products which contain sporiform bacteria such as Bacillus spp.; they could potentially be quite concentrated whereas nonsporiform species (sold in their active form) will necessarily be less concentrated, even if cultivated to carrying capacity. The latter may be centrifuged to varying extents, but I can promise you that such would significantly increase the cost of said product (even while reducing viability).
Also, manufacturer recommendations for dosage might vary even for the same species, or even for the very same product. Some recommended dosages, of course, may be rather arbitrary if not based on what the sales/marketing department believes will keep consumers returning most often (rather than optimal efficacy). Where the dosage is based upon actual research, it can vary significantly depending on intended application. For example, our base dosage of 1.25 ml/gal/day is essentially an average based on numerous peer-reviewed studies where use of PNSB was correlated with improved health of aquacultured species; the key metrics in these investigations, however, range widely from nutrient control, removal of organic wastes, nutrition/probiotic effects, suppression of pathogens, etc.
Since PNSB are completely benign, there's no risk of overdose. In fact, we encourage elevated dosages in certain situations (gut loading live feed, colonizing biomedia, etc.). That being said, many users report satisfactory results at much lower or less frequent doses (it's likely that these are cases where the bacteria are able to persist and even multiply within the system). As such, the 1.25 ml dosage is intended as a starting point that one may safely increase/decrease as desired, though it does appear to be a good dosage for the purpose of supplementing the diets of filter-feeding bacterioplanktivores (including, but not limited to, corals). And that's important, because as far as we're concerned, the primary use of these products is as a nutritious live food. Hence the recommendation for relatively smaller/frequent, rather than big/infrequent, dosages.
Hope that helps!