My guess is that this cropping is being done by video editing service companies at the request of the studios.
Netflix and a studio come to a distribution agreement. Netflix tells the studio the technical requirements for the digital files to send over. The studio realizes they don't have exactly the right format on hand, so they contract out to some firm to encode their content. The studio may not realize that there is an aspect difference, and the service company might automatically adjust the cropping to make it fill the screen to fulfill their contract and meet the technical specifications.
Netflix and a studio come to a distribution agreement. Netflix tells the studio the technical requirements for the digital files to send over. The studio realizes they don't have exactly the right format on hand, so they contract out to some firm to encode their content. The studio may not realize that there is an aspect difference, and the service company might automatically adjust the cropping to make it fill the screen to fulfill their contract and meet the technical specifications.