Sometimes images have an exif data parameter called Orientation with information about how the image should be rotated when displayed. iOS devices read this parameter when displaying images. The WP image rotation tool in the image editor doesn't seem to modify this exif data parameter. So when the image is inserted into a post and viewed on the frontend, it looks fine on Android, but iPhone will rotate the image according to the Orientation parameter.
You can view the parameters an image has at http://exifdata.com