Milk holds vesicles, and it also holds casein micelles of similar size in large numbers. A particle count taken without removing casein first counts both.
Casein micelleVesicle
Labs remove casein before measuring, often with an acid step followed by ultracentrifugation. Those steps help, and they can also change the vesicles: in one bovine milk study, acid treatment partly degraded the surface markers CD9 and CD81.
For a milk- or colostrum-derived ingredient, the method section matters as much as the number. Ask how casein was handled, which markers were tested, and whether the antibodies suit the species.