Back when I drove an Audi, this was a common occurrence. The cars came from the factory with high quality Bosch blades but got replaced at the dealership with low quality "equivalent" blades at the first service. Once the forum community found this out it was an easy matter to look for the absence or presence of the VAG part number on the blade and demand an OEM set at the dealership instead of the cheapy ones.
Not sure if this is the same with BMW but it sounds like it might be, at least at your dealership. Personally I have not experienced this with my car since I've had it.