I don't think it is a huge jump. In the past, I was a high volume seller on Amazon and competitors frequently purchased our products and left unwarranted negative reviews or just left a negative review without ever having purchased it. We knew this because in several cases, we matched the shipment address of our shipment with the return address of a competitors product (this was before FBA was huge).

Plus, sellers wash their hands of this by paying a company to boost their listings. It's don't ask, don't tell on how they accomplish that.