This is because the app has to abide by Apple’s strict content policies, so some profile photos that are fine for the website aren’t able to be displayed on the app. When that happens, our system automatically selects the next-available approved photo to use as your profile photo in the app, and that would be an image that we've already flagged as safe for both the app and for the website.
If you uploaded the photo in the last 24 hours, it's also possible that it has yet to be categorized by our admins (no photo can appear in the app until it's approved as safe) and will appear as your profile photo as soon as it is approved.
If you'd rather have the same photo as your profile photo on the website and the app, you'd have to select a photo that meets our guidelines for safe in both places.
Hope this helps to clarify,
The Grokio Team