Took photos today with iPhone XR. Some were in HEIC format and others were in JPEG but I didnt make any changes to settings. Why differences?

Though capturing in HEIF and HEVC format is recommended, you can set these devices to capture media using the older formats, which are more broadly compatible with other devices and operating systems:

1. Go to Settings > Camera.
2. Tap Formats.
3. Tap Most Compatible. This setting is available only on devices that can capture media in HEIF or HEVC format, and only when using iOS 11 or later.
4. All new photos and videos will now use JPEG or H.264 format. To return to using the space-saving HEIF and HEVC formats, choose High Efficiency.