Just to agree - there is nothing "wrong" here.
If a certain item of content (usu. movies) is "wider" than 16x9 then in order for it to fit a 16x9 display, one of three things needs to occur:
1: It may be stretched vertically to fit. This is never done as it distorts the image, making people and objects disproportionately tall (or thin).
2: It may be enlarged equally vertical and horizontal. Horiziontal enlargement requires the loss of some of the extreme side content.
3: It is displayed with black bars above and below. This is the only "correct" way to show the entire film undistorted and it is these titles that concern us here.
So - how do we display those black bars?
On a regular full HD Blu-Ray BD-Video disc, the image content MUST have a pixel matrix that is 1920W x 1080H. For "wider" content, black bars are added to the encoding process to bring the height up to 1080; they are included in the images on the disc.
For streaming content, this isn't necessary. The (full HD) images in the stream can be 1920W x any height up to 1080. Typically the player software will display the content centrally with nothing (so, black) above and below. Streaming providers may provide content in this way since there is no need, in that context, to do otherwise, and the fact there is no (unnecessary) data associated with black bars saves traffic. Note that this does not compromise the definition of the content; the material part is only ever that central section. The rest is just plain black, and the player will add that as necessary.
It only becomes an issue if one wishes to put such content onto a conforming BD-Video disc. Then, the file will need to be re-encoded, maintaining the 1920 width but adding enough black lines abiove and below to bring the overall shape up to 1080 high.