Actually the quality of whey (amino acid composition, subfractions, and percent ash fat and lactose) can vary widely.
Even when comparing Whey isolates there's a wide variet of techniques companies can use to determine the composition of your protien powder.
There are Ion exchange why isolates (you lose a lot of the subfractions). Ther are a wide variety of more recent processing techniques used to make WPIs-or pull out various subfractions -such as Cross Flow Micro filtration (CFM?), ultra filtration (UF), micro filtration (MF), reverse osmosis (RO), dynamic membrane filtration (DMF), ion exchange chromatography, (IEC), electro-ultrafiltration (EU), radial flow chromatography (RFC) and nano filtration (NF) manufacturers can now make some very high grade and unique whey proteins.
Many manufactures don't go through the trouble to make the high quality isolates. Quality costs and low prices sell.