Why does an image captured in Snapchat look different from the standard iPhone camera app?

Here are two pictures, both taken with an iPhone 6 front facing camera, on the left from the Camera app and on the right from Snapchat.

As you can see, there are two distinct differences:

1. The aspect ratio is different
2. The Camera app correctly mirrors the picture while Snapchat does not

When analysing both pictures with Photo Investigator, here are the resolutions:

• Camera app: 960 x 1280 pixels
• Snapchat: 720 x 1280 pixels

It looks like Snapchat is cropping the picture, most probably so that it could fill the iPhone screen, which makes sense given the purpose of this app.

Out of curiosity, I've tested a few more apps and it's surprising how different their pictures can be:

• Camera+
• Slack
• Google Hangouts
• Instagram
• WhatsApp (as shot and saved)

Even more strangely, WhatsApp correctly mirrors the picture when shooting it but then sends the un-mirrored one via chat.

If you'd like to learn how to correct the mirroring of an image after it has been shot, have a look at another answer of mine on Quora.