What’s up with iPhone’s image numbers?

I tried a couple of scenarios on my device (iPhone 8/iOS 11.2.6), and found the following:

• File numbering is not affected whether my phone is on Wi-Fi or 3G/LTE
• Sequential numbering is also not affected whether you take a picture with the camera app, take a screenshot, or save to your camera roll a photo you receive from messenger apps (SMS/iMessage, Viber or Facebook Messenger)

  the only difference I noticed is the file type -- screenshots are saved as *.png files, while the rest are saved as *.jpg

• The various capture modes of the camera app (photo, portrait, square, panorama) also did not affect the numbering sequence. However, any edit done to these photos will generate a new file with an "E" inserted into the file name -- i.e. "IMG_Exxxx.JPG" -- the xxxx number sequence is still the same.
• The non-sequential numbers I did generate from all this experimentation were from pictures that were:
  • exported by photo/video editor applications (such as Pic Collage)
  • received via air drop from another iOS device (these pictures retained the numbering sequence assigned to it by the transmitting iOS device; no re-naming function done by the receiving iOS device)
• Additionally, videos that you edit and choose the "save as new clip" option will be given a totally random file name, but will retain the original file name assigned if you choose the "trim original" option.

Were those images with random numbers taken by your own phone? Out-of-sequence images are usually from external sources--such as being air dropped from another iOS device or images transferred from a wi-fi enabled camera (such as a GoPro or DSLR).

Sometimes, pictures exported by other apps from within your device may also assign a non-sequential number.