Well one thing I've noticed is that even if I take a photo on my Android phone, I can upload it to my PC and it looks correct on my PC. However when I upload it on the Pizza forum, and it's suddenly rotated. Being OCD, I rotate it (even though it looks sideways on my PC image viewing program) and it then appears fine on the Pizza forum.
So it's not just iPhones. It's happened with my Android phone too.
I think it's just a function of portrait versus landscape and how the various transfers and image processors handle rotated pics.