Why do iPhone photos have Microsoft in Exif data?
"Primary Platform" is part of the ICC profile, which is related to colorspace. Under "Device Model" and "Colorspace", you should see sRGB, which was created jointly by Microsoft and Hewlett-Packard (HP). If you look further you should also see the color profile is listed as being created by, and copyrighted to, HP.

It appears that somewhere along the line, the color profile is being added to the image. It's not clear if this is on iOS (which I've seen claims stating it is not color-managed), during the move to Mac or in iPhoto/Photos. I have images that were in iPhoto and copied back out, and have not been touched by Photos, that have the same Exif data. I have also used the Photo Investigator app on iPhone to view Exif data for images that were taken on iPhone but have not yet been transferred to a computer, and they list the Color Model as RGB, not sRGB, so it would appear the profile is added later.