I mean the overlay is not aligned to the "pixel grid" of the QR code. A QR code is made up of various squares, the size of each square depends on how much data is being embedded into the code. All of these squares have a uniform minimum size for any given QR code. I'm calling the smallest squares "pixels" because they are basically just a single pixel magnified enough to be properly identified by a camera. And the "pixel grid" I'm referring to is just a grid where each cell (or pixel) is the size of these squares.
In the image below, I've outlined a few of these squares ("pixels") to show their size and how some of them are only partially covered by the overlay image.
In the image below, I've outlined the entire area where the overlay covers some of these squares only partially.
In my opinion, QR codes would be more aesthetically pleasing if the overlay image snapped its dimensions to a multiple of the "pixel grid" size, so there are no partial squares showing.