The cameras built into cell phones keep getting better and better, and it is not uncommon to see folks uploading cell phone photos to their WordPress sites. They also have this neat ability to embed the rotation of the phone into the image, so that your photos don't display sideways or upside-down.
But when you use EWWW IO, one of the first things it does is strip the metadata, and now your photos have lost the embedded Orientation flag. To counter that, EWWW IO will first check to see if the Orientation flag is set, and will auto-rotate any image you upload.
This is generally a lossless transformation, but cell phones also have a thing with using odd image dimensions, and the JPG format has this weird thing about using blocks of 16 when rotating an image. If the image boundaries do not fall exactly on a multiple of 16, then some pixels on the edge will be discarded during the rotation. We could leave them on the opposite side of the image, which is what jpegtran likes to do when the image is odd-size, but that's just ugly, so we don't!
Maybe you're not OK with auto-rotation, or maybe you like upside-down photos... In either case, you can disable it by defining EWWW_IMAGE_OPTIMIZER_NO_ROTATE in your wp-config.php file:
define( 'EWWW_IMAGE_OPTIMIZER_NO_ROTATE', true );