It is an understatement to simply say there is a wide variation in quality among dietary supplements. Professional-grade supplements are reliable to the consumer and deliver the intended product without the use of unnecessary fillers, allergens, or other added ingredients. Professional-grade supplements have been tested by an independent 3rd party laboratory to ensure the highest quality of active and inactive ingredients and to ensure each dose delivers the intended amount of active ingredients.
In contrast, standard over the counter vitamins found in grocery stores, health food stores, and chain retail pharmacies are not tested for quality, the accuracy of dose, or presence of contaminants.
Consumer grade products are often not readily absorbed by the body and therefore in general you need about 4 times the amount of a consumer grade product to equal one serving size of a professional grade product.